Biology

Immunity

🛡️ Chapter 15 · MDCAT Biology
📅 Updated Jan 2026 ⏱️ 30 min read 🎯 High-yield

Immunity — Complete MDCAT Notes & MCQs

Full MDCAT Biology Chapter 15 on immunity: innate vs adaptive defense, B and T lymphocytes, antibody structure and classes (IgG, IgM, IgA, IgE, IgD), MHC class I & II, antigen presentation, humoral vs cell-mediated immunity, clonal selection, primary vs secondary responses, active vs passive immunity, autoimmune disorders, SCID, AIDS, hypersensitivity, and vaccination. Includes 200 solved MCQs with explanations and year-wise past papers.

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⚡Quick Answers to Common Searches

Direct, snippet-optimized answers to the top questions students search about Immunity.

What is immunity?

Immunity is the body's ability to resist and defend against pathogens and foreign substances via innate (non-specific) and adaptive (specific) mechanisms.

Innate vs adaptive immunity?

Innate: non-specific, fast, no memory (skin, phagocytes, inflammation). Adaptive: specific, slower, has memory (B and T lymphocytes).

4 characteristics of adaptive immunity?

Specificity, Diversity, Memory, and Self-tolerance.

B cells vs T cells?

B cells mature in bone marrow, mediate humoral immunity. T cells mature in thymus, mediate cell-mediated immunity.

Structure of antibody?

Y-shaped protein with 2 heavy + 2 light chains. Variable regions bind antigen; constant regions determine class/function.

5 antibody classes?

IgG (most abundant, placenta), IgM (pentamer, first), IgA (secretions), IgE (allergy), IgD (BCR).

MHC I vs MHC II?

MHC I: all nucleated cells, presents to CD8+ T cells. MHC II: professional APCs, presents to CD4+ T cells.

Humoral vs cell-mediated?

Humoral: B cell antibodies, extracellular pathogens. Cell-mediated: T cells, intracellular pathogens, cancer, transplants.

What is clonal selection?

Antigen activates only the matching lymphocyte clone, causing it to multiply into effector and memory cells.

Primary vs secondary response?

Primary: 3-14 day lag, IgM then IgG, generates memory. Secondary: 1-3 day lag, faster, stronger, higher-affinity IgG.

4 types of acquired immunity?

Active natural (infection), Active artificial (vaccination), Passive natural (maternal), Passive artificial (antivenom).

What is a vaccine?

Biological preparation providing active artificial immunity by stimulating immune response without causing disease.

AIDS & immune system?

HIV attacks CD4+ helper T cells. Loss of helper T cells cripples both humoral and cell-mediated immunity.

What is autoimmune disease?

Immune system attacks self-tissues (rheumatoid arthritis, Type I diabetes, multiple sclerosis) due to loss of self-tolerance.

4 hypersensitivity types?

Type I (IgE, allergy), Type II (antibody-mediated), Type III (immune complex), Type IV (delayed, T cell).

❓People Also Ask — Immunity

Questions students most frequently search about this chapter. Answers are direct and exam-focused.

QWhat is the difference between innate and adaptive immunity?
Innate immunity is non-specific, fast, has no memory (skin, phagocytes, inflammation, complement). Adaptive immunity is specific, slower, has memory (B and T lymphocytes).
QWhat are the 4 characteristics of adaptive immunity?
Specificity (recognizes unique antigens), Diversity (vast receptor repertoire), Memory (rapid secondary response), and Self-tolerance (avoids attacking self).
QWhat are the functions of Helper T cells and Cytotoxic T cells?
Helper T cells (CD4+) secrete cytokines that activate B cells, cytotoxic T cells, and macrophages. Cytotoxic T cells (CD8+) kill infected or cancerous cells using perforin and granzymes.
QWhat is the function of Regulatory T cells (Tregs)?
Regulatory T cells (FOXP3+) suppress immune responses and maintain self-tolerance. Deficiency causes autoimmune disease.
QWhat is the difference between MHC class I and MHC class II?
MHC class I is on all nucleated cells, presents endogenous antigens to CD8+ T cells. MHC class II is on professional APCs (dendritic cells, macrophages, B cells), presents exogenous antigens to CD4+ T cells.
QWhat is the difference between a hapten and an antigen?
An antigen is a complete molecule that elicits an immune response. A hapten is a small molecule that is NOT immunogenic alone but becomes immunogenic when attached to a carrier protein.
QWhat is opsonization?
Opsonization is the coating of a pathogen with antibodies or complement (C3b), enhancing phagocytosis by macrophages and neutrophils.
QWhat is the difference between neutralization and agglutination?
Neutralization: antibodies block toxin or viral activity. Agglutination: antibodies cross-link pathogens into clumps, aiding phagocytosis.
QWhat is the Complement system?
The complement system is a cascade of proteins (C1-C9) that opsonize pathogens, attract phagocytes (C3a, C5a), and form the Membrane Attack Complex (MAC) to lyse cells.
QWhat is clonal selection theory?
Clonal selection describes how an antigen selectively activates only the lymphocytes bearing the matching receptor, causing them to multiply into a large clone of effector and memory cells.
QWhy does the secondary immune response produce predominantly IgG?
After the primary response, memory B cells have undergone class switching (IgM → IgG) and affinity maturation, so on re-exposure they rapidly produce high-affinity IgG.
QWhich immunoglobulin crosses the placenta?
Only IgG crosses the placenta, via the neonatal Fc receptor (FcRn), providing passive immunity to the fetus.
QWhat is the role of the J chain?
The J chain links monomers of IgA (forming dimers) and IgM (forming pentamers), allowing these antibodies to polymerize.
QWhat is the difference between active and passive immunity?
Active: body produces its own antibodies (from infection or vaccination), long-lasting, has memory. Passive: preformed antibodies are given, immediate but temporary, no memory.
QWhat is a toxoid vaccine?
A toxoid is an inactivated bacterial toxin (e.g., tetanus, diphtheria) that retains immunogenicity but has lost toxicity. It produces active artificial immunity.
QWhat is SCID?
SCID (Severe Combined Immunodeficiency) is a genetic disorder involving defective development of both B and T cells. Patients are extremely vulnerable to infections.
QHow does HIV cause immunodeficiency?
HIV infects and destroys CD4+ helper T cells. Their loss cripples both cell-mediated and humoral immunity, leading to AIDS and opportunistic infections.
QWhat are the 4 types of hypersensitivity?
Type I: IgE-mediated (allergy). Type II: antibody-mediated (cytotoxic). Type III: immune complex. Type IV: cell-mediated (delayed, T cell).
QWhy is graft rejection primarily mediated by T cells?
T cells recognize foreign MHC molecules on donor tissue as non-self, triggering direct and indirect allorecognition, activation, and destruction of the graft.
QWhat are monoclonal antibodies?
Monoclonal antibodies are identical antibodies derived from a single B cell clone (often via hybridoma technology). They are used in diagnostics, cancer therapy, and autoimmune treatment.

📋Syllabus & Learning Outcomes

According to PMDC/PMC MDCAT syllabus, Immunity covers:

📝Full Notes — Immunity

🛡️ Immunity – Specific Defense Mechanisms

The immune system defends the body against pathogens through innate (non-specific) and adaptive (specific) mechanisms. Adaptive immunity provides targeted, long-lasting protection with memory.

🧬 Adaptive Immunity – Key Features

  • Specificity: lymphocytes recognize unique antigens via specific receptors.
  • Diversity: vast repertoire of receptors generated by V(D)J recombination.
  • Memory: memory cells enable rapid secondary (anamnestic) responses.
  • Self-Tolerance: ability to distinguish self from non-self; failure leads to autoimmunity.

🦴 Lymphocytes – B Cells and T Cells

  • B lymphocytes: mature in bone marrow. Mediate humoral immunity (antibody production).
  • T lymphocytes: mature in thymus. Mediate cell-mediated immunity.
    • Helper T cells (CD4+): activate B cells, cytotoxic T cells, and macrophages via cytokines.
    • Cytotoxic T cells (CD8+): kill infected/cancer cells using perforin and granzymes.
    • Regulatory T cells (Tregs, FOXP3+): suppress immune responses, maintain tolerance.

🧪 Antigens and Antibodies

  • Antigen: any molecule (usually protein or polysaccharide) that elicits an immune response.
  • Epitope (antigenic determinant): specific part of antigen recognized by antibody or TCR.
  • Antibody (Immunoglobulin, Ig): Y-shaped protein with two heavy and two light chains.
    • Variable region (Fab): binds antigen; contains CDRs for specificity.
    • Constant region (Fc): determines isotype and effector function.
    • Hinge region: provides flexibility for bivalent binding.

🧪 Antibody Classes

ClassStructureKey Functions
IgGMonomerMost abundant (75%); crosses placenta; secondary response; opsonization
IgMPentamerFirst produced in primary response; activates complement
IgADimer (secretory)Mucosal immunity (tears, saliva, colostrum, gut)
IgEMonomerAllergy; defense against parasites (binds mast cells)
IgDMonomerB cell receptor; function in serum unclear

🔄 Antigen Presentation and MHC

  • MHC Class I: on all nucleated cells. Presents endogenous antigens to CD8+ T cells.
  • MHC Class II: on professional APCs (dendritic cells, macrophages, B cells). Presents exogenous antigens to CD4+ T cells.
  • Antigen Presenting Cells (APCs): process and present antigens on MHC molecules with co-stimulatory signals.

⚔️ Humoral vs. Cell-Mediated Immunity

  • Humoral (B cell): antibodies neutralize toxins, opsonize bacteria, activate complement. Effective against extracellular pathogens.
  • Cell-Mediated (T cell): cytotoxic T cells kill infected cells; helper T cells orchestrate responses. Effective against intracellular pathogens, viruses, cancer, transplants.

📈 Immune Responses

  • Primary response: lag phase (3-14 days); IgM then IgG; generates memory cells.
  • Secondary (anamnestic) response: faster, stronger, higher affinity IgG due to memory cells and affinity maturation.

💉 Types of Acquired Immunity

  • Active Natural: infection → own immune response → memory.
  • Active Artificial: vaccination → own immune response → memory.
  • Passive Natural: maternal IgG across placenta; IgA in breast milk.
  • Passive Artificial: injection of preformed antibodies (e.g., antivenom, TIG).

🩺 Disorders and Clinical Correlates

  • Autoimmunity: immune system attacks self (e.g., rheumatoid arthritis, Type I diabetes).
  • Immunodeficiency: SCID (lack of B and T cells), AIDS (HIV destroys CD4+ T cells).
  • Hypersensitivity: Type I (IgE, allergy), Type II (cytotoxic), Type III (immune complex), Type IV (delayed, T cell).
  • Graft rejection: T cell-mediated response to foreign MHC.

💡 Pro tip: Know the antibody classes, their functions, and the differences between humoral and cell-mediated immunity. Understand the primary vs. secondary response curves.

✅Practice MCQs — Test 1

🧪 Basic Concepts

50 MCQs
1Which of the following is considered the third line of defense?
  • A. Skin
  • B. Inflammation
  • C. Specific immunity
  • D. Phagocytosis
Answer: C. Specific immunity
Specific (adaptive) immunity is the third line of defense, providing targeted responses.
2Specificity in immunity is primarily due to:
  • A. Neutrophils
  • B. Lymphocytes
  • C. Macrophages
  • D. Natural killer cells
Answer: B. Lymphocytes
Lymphocytes (B and T cells) have unique receptors that recognize specific antigens.
3The ability of the immune system to recognize millions of different antigens is called:
  • A. Specificity
  • B. Memory
  • C. Diversity
  • D. Tolerance
Answer: C. Diversity
Diversity refers to the vast repertoire of antigen receptors generated by genetic recombination.
4Antigens are usually chemically composed of:
  • A. Large proteins or polysaccharides
  • B. Simple lipids
  • C. Small glucose molecules
  • D. Inorganic salts
Answer: A
Most antigens are large, complex molecules, typically proteins or polysaccharides.
5B-lymphocytes are named 'B' because they mature in the:
  • A. Blood
  • B. Bursa of Fabricius/Bone marrow
  • C. Brain
  • D. Basophils
Answer: B
In mammals, B cells mature in bone marrow; in birds, in the bursa of Fabricius.
6T-lymphocytes undergo maturation and differentiation in the:
  • A. Thyroid
  • B. Thymus
  • C. Tonsils
  • D. Tendons
Answer: B. Thymus
T cells mature in the thymus gland.
7Which cells are responsible for humoral immunity?
  • A. T-cells
  • B. B-cells
  • C. Neutrophils
  • D. Erythrocytes
Answer: B. B-cells
B cells produce antibodies, mediating humoral immunity.
8Humoral immunity involves the production of:
  • A. Granzymes
  • B. Perforins
  • C. Antibodies
  • D. Histamines
Answer: C. Antibodies
Humoral immunity is antibody-mediated.
9Cell-mediated immunity is primarily the function of:
  • A. Plasma cells
  • B. T-lymphocytes
  • C. Platelets
  • D. Mast cells
Answer: B. T-lymphocytes
T cells (cytotoxic, helper, regulatory) mediate cellular immunity.
10Which T-cell type acts as the 'manager' of the immune system by activating other cells?
  • A. Cytotoxic T-cells
  • B. Helper T-cells
  • C. Suppressor T-cells
  • D. Memory T-cells
Answer: B. Helper T-cells
Helper T cells (CD4+) secrete cytokines that activate B cells, cytotoxic T cells, and macrophages.
11Cytotoxic T-cells recognize antigens presented by:
  • A. MHC-I
  • B. MHC-II
  • C. Antibodies
  • D. Red blood cells
Answer: A. MHC-I
CD8+ T cells recognize endogenous antigens on MHC class I molecules.
12The primary function of Plasma cells is to:
  • A. Phagocytose bacteria
  • B. Secrete antibodies
  • C. Kill virus-infected cells
  • D. Produce bile
Answer: B. Secrete antibodies
Plasma cells are differentiated B cells that produce and secrete large quantities of antibodies.
13Immunoglobulins is another name for:
  • A. Antigens
  • B. Antibodies
  • C. Hormones
  • D. Enzymes
Answer: B. Antibodies
Immunoglobulins (Ig) are antibodies.
14The basic structure of an antibody molecule is:
  • A. X-shaped
  • B. Y-shaped
  • C. Circular
  • D. Linear
Answer: B. Y-shaped
Antibodies have a characteristic Y-shaped structure.
15An antibody molecule consists of how many polypeptide chains?
  • A. Two
  • B. Three
  • C. Four
  • D. Five
Answer: C. Four
A monomeric antibody has two heavy and two light chains (total four polypeptides).
16The region of the antibody that binds to the antigen is called the:
  • A. Constant region
  • B. Variable region
  • C. Hinge region
  • D. Heavy chain
Answer: B. Variable region
The variable regions at the tips of the Y determine antigen specificity.
17Which immunoglobulin is most abundant in human serum?
  • A. IgA
  • B. IgE
  • C. IgG
  • D. IgM
Answer: C. IgG
IgG constitutes about 75-80% of serum immunoglobulins.
18Which antibody is found in secretions like tears and colostrum?
  • A. IgD
  • B. IgA
  • C. IgG
  • D. IgE
Answer: B. IgA
Secretory IgA is present in mucosal secretions and colostrum.
19The type of immunity acquired by a fetus from its mother through the placenta is:
  • A. Active natural
  • B. Passive natural
  • C. Active artificial
  • D. Passive artificial
Answer: B. Passive natural
Maternal IgG crosses the placenta, providing passive natural immunity.
20Vaccination provides which type of immunity?
  • A. Active artificial
  • B. Passive artificial
  • C. Passive natural
  • D. Innate
Answer: A. Active artificial
Vaccines stimulate the body's own immune response, producing active artificial immunity.
21The faster and stronger response to a second exposure to the same antigen is due to:
  • A. Plasma cells
  • B. Memory cells
  • C. Neutrophils
  • D. Helper T-cells
Answer: B. Memory cells
Memory B and T cells enable a rapid secondary (anamnestic) response.
22Major Histocompatibility Complex (MHC) proteins are essential for:
  • A. Nutrient transport
  • B. Self/non-self recognition
  • C. Blood clotting
  • D. Oxygen binding
Answer: B. Self/non-self recognition
MHC molecules present peptide antigens to T cells for immune recognition.
23Which cells destroy infected cells by making holes in their cell membranes?
  • A. B-cells
  • B. Helper T-cells
  • C. Cytotoxic T-cells
  • D. Suppressor T-cells
Answer: C. Cytotoxic T-cells
Cytotoxic T cells release perforin and granzymes to kill infected cells.
24The proteins used by T-cells to create pores in target cells are called:
  • A. Antibodies
  • B. Perforins
  • C. Interferons
  • D. Complement proteins
Answer: B. Perforins
Perforin polymerizes to form pores in the target cell membrane.
25Autoimmune diseases occur when the immune system:
  • A. Is too weak to fight infection
  • B. Fails to distinguish self from non-self
  • C. Overreacts to pollen
  • D. Stops producing white blood cells
Answer: B
Autoimmunity results from loss of self-tolerance.
26AIDS is caused by a virus that specifically attacks:
  • A. B-cells
  • B. Helper T-cells
  • C. Red blood cells
  • D. Muscle cells
Answer: B. Helper T-cells
HIV targets CD4+ helper T cells, leading to immunodeficiency.
27The binding of an antibody to a toxin to neutralize it is called:
  • A. Agglutination
  • B. Opsonization
  • C. Neutralization
  • D. Precipitation
Answer: C. Neutralization
Neutralization blocks the toxic activity of pathogens or toxins.
28Clumping of bacteria by antibodies is known as:
  • A. Agglutination
  • B. Lysis
  • C. Inflammation
  • D. Digestion
Answer: A. Agglutination
Agglutination clumps particulate antigens, aiding phagocytosis.
29Which of the following is a primary lymphoid organ?
  • A. Spleen
  • B. Lymph node
  • C. Thymus
  • D. Tonsils
Answer: C. Thymus
Primary lymphoid organs (bone marrow and thymus) are where lymphocytes mature.
30Secondary immune response is characterized by a high titer of which antibody?
  • A. IgM
  • B. IgD
  • C. IgG
  • D. IgE
Answer: C. IgG
Secondary response produces predominantly IgG due to class switching.
31The short-lived cells that respond immediately to an antigen are:
  • A. Memory cells
  • B. Effector cells
  • C. Stem cells
  • D. Fibroblasts
Answer: B. Effector cells
Effector cells (plasma cells, cytotoxic T cells) act during the primary response.
32How many binding sites does a typical monomeric antibody have?
  • A. One
  • B. Two
  • C. Four
  • D. Ten
Answer: B. Two
Monomeric IgG has two identical antigen-binding sites.
33Which chain of the antibody molecule has a lower molecular weight?
  • A. Heavy chain
  • B. Light chain
  • C. Alpha chain
  • D. Beta chain
Answer: B. Light chain
Light chains (~25 kDa) are smaller than heavy chains (~50 kDa).
34T-cells are involved in:
  • A. Graft rejection
  • B. Allergy to pollen
  • C. Antibody secretion
  • D. Both A and B
Answer: D
T cells mediate graft rejection and also help in allergic responses (Type IV).
35Passive immunity is used for:
  • A. Long-term protection
  • B. Immediate treatment (e.g., snake bite)
  • C. Routine childhood vaccination
  • D. Memory cell production
Answer: B
Passive immunity provides immediate but temporary protection.
36An example of passive artificial immunity is:
  • A. Getting measles
  • B. Breastfeeding
  • C. Antivenom injection
  • D. Polio drops
Answer: C. Antivenom injection
Antivenom contains preformed antibodies, giving artificial passive immunity.
37What is the lag period in a primary immune response?
  • A. 0 hours
  • B. 3–14 days
  • C. 1 month
  • D. 1 year
Answer: B. 3–14 days
The lag phase lasts several days while clonal expansion occurs.
38The specific part of the antigen that is recognized by the antibody is the:
  • A. Paratope
  • B. Epitope
  • C. Isotope
  • D. Clone
Answer: B. Epitope
Epitope (antigenic determinant) binds to the antibody's paratope.
39Helper T-cells are also known as:
  • A. CD4+ cells
  • B. CD8+ cells
  • C. B-cells
  • D. Macrophages
Answer: A. CD4+ cells
Helper T cells express CD4 glycoprotein.
40Cytotoxic T-cells are also known as:
  • A. CD4+ cells
  • B. CD8+ cells
  • C. Plasma cells
  • D. Natural killer cells
Answer: B. CD8+ cells
Cytotoxic T cells express CD8.
41Which of the following is NOT a feature of adaptive immunity?
  • A. Memory
  • B. Rapid non-specific response
  • C. Specificity
  • D. Self-tolerance
Answer: B
Rapid non-specific response is a feature of innate immunity.
42The process of coating a pathogen to enhance phagocytosis is:
  • A. Neutralization
  • B. Opsonization
  • C. Lysis
  • D. Agglutination
Answer: B. Opsonization
Opsonins (antibodies or complement) tag pathogens for phagocytosis.
43Which cells remain in the body for years to fight future infections?
  • A. Plasma cells
  • B. Memory cells
  • C. Mast cells
  • D. Eosinophils
Answer: B. Memory cells
Memory B and T cells provide long-term immunological memory.
44Active immunity involves the production of:
  • A. Antigens
  • B. Antibodies by the individual's own body
  • C. Antibodies by a donor
  • D. Only physical barriers
Answer: B
Active immunity results from one's own immune response.
45The variable region of an antibody is found at:
  • A. The stem of the Y
  • B. The tips of the Y
  • C. The hinge only
  • D. The entire light chain
Answer: B. The tips of the Y
Variable domains are at the N-terminal ends of both heavy and light chains.
46Interleukins are chemical messengers produced by:
  • A. Red blood cells
  • B. Helper T-cells
  • C. Platelets
  • D. Osteocytes
Answer: B. Helper T-cells
Helper T cells produce interleukins (cytokines) that regulate immune responses.
47The 'Dual path' of the immune system refers to:
  • A. Skin and mucous
  • B. Humoral and Cell-mediated
  • C. Arteries and veins
  • D. Brain and spinal cord
Answer: B
Adaptive immunity has two branches: humoral (antibody) and cell-mediated.
48Which organ filters the blood and traps blood-borne antigens?
  • A. Lymph nodes
  • B. Spleen
  • C. Liver
  • D. Heart
Answer: B. Spleen
The spleen filters blood and mounts immune responses to blood-borne antigens.
49Clonal selection theory explains:
  • A. How blood clots
  • B. How specific lymphocytes multiply in response to an antigen
  • C. How the heart beats
  • D. How nerves conduct impulses
Answer: B
Clonal selection describes antigen-driven expansion of specific lymphocyte clones.
50Suppressor T-cells function to:
  • A. Attack viruses
  • B. Turn off the immune response once the threat is gone
  • C. Produce more antibodies
  • D. Increase inflammation
Answer: B
Regulatory T cells suppress immune responses, maintaining tolerance.

✅Practice MCQs — Test 2

🧪 Advanced Concepts

50 MCQs
1The specific site on an antigen where the paratope of an antibody binds is the epitope; however, if an antigen has multiple epitopes of different types, it is termed:
  • A. Monovalent
  • B. Multivalent
  • C. Hapten
  • D. Adjuvant
Answer: B. Multivalent
Multivalent antigens have multiple different epitopes.
2Which of the following best describes a 'Hapten'?
  • A. A complete antigen
  • B. A protein that triggers a fever
  • C. A small molecule that is only immunogenic when attached to a carrier protein
  • D. A type of cytokine
Answer: C
Haptens are small molecules that require a carrier to become immunogenic.
3During B-cell activation, the process where the affinity of antibodies for an antigen increases with prolonged exposure is called:
  • A. Somatic hypermutation
  • B. Clonal deletion
  • C. Isotype switching
  • D. Opsonization
Answer: A. Somatic hypermutation
Somatic hypermutation generates higher affinity B cell receptors, leading to affinity maturation.
4Which MHC class molecule is found on almost all nucleated cells in the human body?
  • A. MHC-I
  • B. MHC-II
  • C. MHC-III
  • D. CD4
Answer: A. MHC-I
MHC class I is expressed on all nucleated cells.
5MHC-II molecules are strictly restricted to:
  • A. Neurons
  • B. Professional Antigen Presenting Cells (APCs)
  • C. Red blood cells
  • D. Muscle fibers
Answer: B. Professional APCs
MHC II is found on dendritic cells, macrophages, and B cells.
6The 'Two-Signal' hypothesis for T-cell activation requires the recognition of the Antigen-MHC complex and:
  • A. Histamine release
  • B. Co-stimulatory signals (e.g., B7-CD28)
  • C. Antibody binding
  • D. Passive diffusion
Answer: B
Signal 1 is TCR-MHC/peptide; Signal 2 is CD28-B7 costimulation.
7Which enzyme is released by Cytotoxic T-cells to induce apoptosis (programmed cell death) in a target cell?
  • A. Lysozyme
  • B. Amylase
  • C. Granzyme
  • D. Reverse transcriptase
Answer: C. Granzyme
Granzymes enter target cells and activate caspases, triggering apoptosis.
8The phenomenon where a B-cell changes the constant region of its antibody from IgM to IgG without changing the variable region is:
  • A. VDJ recombination
  • B. Class switching
  • C. Agglutination
  • D. Gene silencing
Answer: B. Class switching
Isotype switching alters effector function while preserving antigen specificity.
9Which of the following is a 'secondary' lymphoid organ where the actual immune encounter typically occurs?
  • A. Bone marrow
  • B. Thymus
  • C. Peyer's patches
  • D. Thyroid
Answer: C. Peyer's patches
Peyer's patches (GALT) are secondary lymphoid tissues.
10What is the functional role of the 'J-chain' in antibody structure?
  • A. It binds to the antigen
  • B. It links monomers of IgA or IgM into polymers
  • C. It determines the idiotype
  • D. It activates the complement system
Answer: B
J chain joins polymeric IgA and IgM.
11In the classical pathway of the Complement System, which antibody classes are most effective at activation?
  • A. IgE and IgD
  • B. IgG and IgM
  • C. IgA and IgE
  • D. IgG and IgA
Answer: B. IgG and IgM
IgG and IgM activate classical complement pathway.
12Dendritic cells are considered the most potent APCs because they:
  • A. Produce antibodies
  • B. Are the only cells with MHC-I
  • C. Express high levels of MHC-II and co-stimulatory molecules
  • D. Can live forever
Answer: C
High expression of MHC II and co-stimulators makes dendritic cells efficient.
13The TCR (T-Cell Receptor) differs from a B-cell receptor (antibody) in that:
  • A. TCR is secreted
  • B. TCR only recognizes processed peptides presented on MHC
  • C. TCR has four binding sites
  • D. TCR recognizes lipids only
Answer: B
TCR recognizes peptide-MHC complexes, not native antigens.
14Which cytokine is primarily responsible for the autocrine proliferation of T-cells after activation?
  • A. Interleukin-1
  • B. Interleukin-2
  • C. Interferon-gamma
  • D. Tumor Necrosis Factor
Answer: B. Interleukin-2
IL-2 drives T cell clonal expansion.
15The heavy chain of an IgG molecule is specifically called the:
  • A. Mu chain
  • B. Gamma chain
  • C. Alpha chain
  • D. Delta chain
Answer: B. Gamma chain
IgG heavy chain is γ (gamma).
16Which region of the antibody determines its 'Isotype'?
  • A. Variable region of Light chain
  • B. Constant region of Heavy chain
  • C. Hypervariable region
  • D. The hinge region only
Answer: B
The constant region of the heavy chain defines the class (isotype).
17A person with a genetic defect preventing the expression of MHC-II would specifically lack:
  • A. Innate immunity
  • B. CD8+ T-cell activity
  • C. CD4+ T-cell activity
  • D. Natural Killer cells
Answer: C
MHC II is required for CD4+ T cell activation.
18Which of the following is an example of 'Artificial Passive Immunity'?
  • A. Recovery from mumps
  • B. Administration of Tetanus Immunoglobulin (TIG)
  • C. Oral Polio Vaccine (OPV)
  • D. Injection of a weakened toxin
Answer: B
TIG contains preformed antibodies, giving artificial passive immunity.
19The process of 'Clonal Deletion' in the thymus is essential for:
  • A. Fighting viruses
  • B. Eliminating self-reactive T-cells
  • C. Increasing antibody diversity
  • D. Activating macrophages
Answer: B
Negative selection eliminates T cells that strongly recognize self-peptides.
20Which antibody type is a pentamer in its secreted form?
  • A. IgG
  • B. IgA
  • C. IgM
  • D. IgE
Answer: C. IgM
Secreted IgM is a pentamer held by J chain.
21T-independent antigens (like bacterial polysaccharides) can activate:
  • A. T-cells directly
  • B. B-cells without Helper T-cell assistance
  • C. Only the skin
  • D. Memory T-cells
Answer: B
T-independent antigens stimulate B cells directly, without T cell help.
22The 'Hinge Region' of an antibody provides:
  • A. Antigen specificity
  • B. Flexibility to bind to epitopes at varying distances
  • C. Protection against stomach acid
  • D. The site for J-chain attachment
Answer: B
The hinge allows flexibility for simultaneous binding to two epitopes.
23Monoclonal antibodies are derived from:
  • A. Multiple B-cell lineages
  • B. A single clone of a hybridoma cell
  • C. T-cell clusters
  • D. Serum of an infected animal
Answer: B
Hybridomas produce identical antibodies from a single clone.
24What occurs during the 'Lag Phase' of a primary immune response?
  • A. Massive antibody production
  • B. Selection and clonal expansion of specific lymphocytes
  • C. Immediate destruction of all pathogens
  • D. Permanent immunity is achieved
Answer: B
The lag phase involves activation and clonal expansion before antibody detection.
25An 'Anamnestic Response' refers to:
  • A. The first time the body sees a germ
  • B. The rapid secondary immune response
  • C. Allergic reaction to dust
  • D. Failure of the immune system
Answer: B
Anamnestic (secondary) response is faster and stronger due to memory cells.
26Which molecule acts as a bridge between the innate and adaptive immune systems?
  • A. Hemoglobin
  • B. Complement C3b (acting as an opsonin)
  • C. Keratin
  • D. Insulin
Answer: B
C3b links innate complement activation to adaptive opsonization.
27Th1 cells primarily assist in:
  • A. Humoral immunity
  • B. Cell-mediated immunity and macrophage activation
  • C. Allergic reactions
  • D. Suppressing the immune system
Answer: B
Th1 cells secrete IFN-γ, activating macrophages and cytotoxic T cells.
28Th2 cells are mainly involved in:
  • A. Activating B-cells for antibody production
  • B. Killing cancer cells
  • C. Producing stomach acid
  • D. Bone marrow formation
Answer: A
Th2 cells help B cells produce antibodies, especially IgE.
29The specific recombination of gene segments (V, D, J) to create diversity occurs in:
  • A. Mature plasma cells
  • B. Developing lymphocytes in primary lymphoid organs
  • C. The spleen during infection
  • D. Red blood cells
Answer: B
VDJ recombination occurs during lymphocyte development in bone marrow and thymus.
30Which of the following can cross the human placenta?
  • A. IgM
  • B. IgA
  • C. IgG
  • D. IgD
Answer: C. IgG
Only IgG crosses the placenta via FcRn receptors.
31The 'Membrane Attack Complex' (MAC) is the end-product of:
  • A. B-cell activation
  • B. The Complement Cascade
  • C. T-cell maturation
  • D. Phagocytosis
Answer: B. The Complement Cascade
MAC (C5b-9) forms pores in target cell membranes.
32If a T-cell recognizes an MHC-antigen complex without a co-stimulatory signal, it enters a state of:
  • A. Hyperactivity
  • B. Anergy (unresponsiveness)
  • C. Immediate mitosis
  • D. Mutation
Answer: B. Anergy
Lack of costimulation induces anergy, preventing autoimmunity.
33Which antibody mediates Type I hypersensitivity (allergic) reactions?
  • A. IgG
  • B. IgM
  • C. IgE
  • D. IgA
Answer: C. IgE
IgE binds to mast cells and triggers histamine release.
34The diversity of antigen receptors is generated before or after exposure to the antigen?
  • A. Before (Antigen-independent)
  • B. After (Antigen-dependent)
  • C. Only during old age
  • D. Only during pregnancy
Answer: A
Receptor diversity is generated by genetic recombination before antigen encounter.
35SCID (Severe Combined Immunodeficiency) is characterized by a lack of:
  • A. Only B-cells
  • B. Only T-cells
  • C. Both B and T-lymphocytes
  • D. Only Neutrophils
Answer: C
SCID involves defective development of both B and T cells.
36Which cell type links the adaptive response to helminth (worm) infections?
  • A. Neutrophils
  • B. Eosinophils (via IgE)
  • C. Basophils
  • D. Erythrocytes
Answer: B. Eosinophils
Eosinophils kill helminths via IgE-mediated ADCC.
37The 'Constant Region' of an antibody (Fc) binds to:
  • A. The Epitope
  • B. Receptors on phagocytes or complement proteins
  • C. The Paratope
  • D. The light chain
Answer: B
Fc region mediates effector functions like opsonization and complement activation.
38Endogenous antigens (produced inside the cell) are presented via:
  • A. MHC-I
  • B. MHC-II
  • C. CD4
  • D. Surface IgD
Answer: A. MHC-I
Endogenous antigens (e.g., viral proteins) are presented on MHC I.
39Exogenous antigens (taken in from outside) are presented via:
  • A. MHC-I
  • B. MHC-II
  • C. CD8
  • D. Mitochondria
Answer: B. MHC-II
Exogenous antigens are processed and presented on MHC II.
40Which of these is a 'Cytokine Storm'?
  • A. A healthy immune response
  • B. A systemic, overproduced inflammatory signaling
  • C. Lack of white blood cells
  • D. High production of Red blood cells
Answer: B
Cytokine storm is a dangerous hyperinflammatory state.
41The idiotype of an antibody is determined by:
  • A. The Heavy chain constant region
  • B. The hypervariable regions of both H and L chains
  • C. The J-chain
  • D. The number of disulfide bonds
Answer: B
Idiotype is the unique antigen-binding site formed by variable regions.
42'T-cell exhaustion' refers to:
  • A. Physical tiredness
  • B. Loss of effector function during chronic infection
  • C. High energy levels of T-cells
  • D. Rapid movement of T-cells
Answer: B
Exhausted T cells lose function in chronic infections like HIV or cancer.
43Which molecule on a Cytotoxic T-cell binds to the MHC-I molecule?
  • A. CD4
  • B. CD8
  • C. CD28
  • D. gp120
Answer: B. CD8
CD8 coreceptor binds MHC class I.
44Which molecule on a Helper T-cell binds to the MHC-II molecule?
  • A. CD4
  • B. CD8
  • C. IgG
  • D. BCR
Answer: A. CD4
CD4 coreceptor binds MHC class II.
45Active immunity is characterized by:
  • A. Short-term protection
  • B. Presence of memory cells
  • C. Direct injection of antibodies
  • D. Immediate effect within seconds
Answer: B
Active immunity generates memory cells for long-term protection.
46The process by which the immune system ignores the body's own proteins is:
  • A. Tolerance
  • B. Sensitization
  • C. Opsonization
  • D. Agglutination
Answer: A. Tolerance
Self-tolerance prevents autoimmune reactions.
47Which cells are primarily responsible for 'Graft vs Host' disease?
  • A. B-cells
  • B. T-cells
  • C. Platelets
  • D. Macrophages
Answer: B. T-cells
Donor T cells attack recipient tissues in GVHD.
48Most T-cells in the peripheral circulation are:
  • A. Double positive (CD4+ CD8+)
  • B. Single positive (either CD4+ or CD8+)
  • C. Double negative
  • D. Lacking a nucleus
Answer: B
Mature T cells express either CD4 or CD8, not both.
49In a secondary immune response, the 'Affinity' of the antibodies is generally:
  • A. Lower than the primary response
  • B. Higher than the primary response
  • C. The same as the primary response
  • D. Zero
Answer: B
Affinity maturation produces higher affinity antibodies in secondary responses.
50What defines the specificity of the adaptive immune system?
  • A. The presence of skin
  • B. The unique shape of the antigen-binding site on receptors
  • C. The speed of the heartbeat
  • D. The level of calcium in the blood
Answer: B
Specificity is determined by unique receptors on lymphocytes.

✅Practice MCQs — Test 3

🧪 Exam-Level Questions

50 MCQs
1Enzymatic cleavage of an IgG molecule by papain specifically yields:
  • A. One F(ab')2 fragment and fragmented Fc
  • B. Two Fab fragments and one intact Fc fragment
  • C. Two Fc fragments and one Fab
  • D. Complete degradation into amino acids
Answer: B
Papain cuts above the hinge, producing two Fab and one Fc fragment.
2The Transporter Associated with Antigen Processing (TAP) protein complex is crucial for transporting cytosolic peptides into the rough endoplasmic reticulum to be loaded onto:
  • A. MHC-II molecules
  • B. MHC-I molecules
  • C. T-Cell Receptors
  • D. B-Cell Receptors
Answer: B
TAP transports peptides for loading onto MHC class I.
3In the exogenous antigen processing pathway, premature binding of endogenous peptides to the MHC-II groove in the ER is prevented by the:
  • A. J-chain
  • B. Invariant chain (Ii)
  • C. Beta-2 microglobulin
  • D. Perforin
Answer: B
Invariant chain (CD74) blocks the peptide-binding groove of MHC II.
4The generation of massive antibody diversity occurs largely prior to antigen exposure through a process involving RAG-1 and RAG-2 enzymes known as:
  • A. Isotype switching
  • B. V(D)J recombination
  • C. Somatic hypermutation
  • D. Clonal expansion
Answer: B. V(D)J recombination
VDJ recombination assembles variable region exons from gene segments.
5To undergo T-cell dependent isotype switching (e.g., from IgM to IgG), B-cells require a co-stimulatory signal mediated by the interaction of CD40 on the B-cell with which molecule on the Helper T-cell?
  • A. CD28
  • B. B7-1/B7-2
  • C. CD40 Ligand (CD40L)
  • D. CTLA-4
Answer: C
CD40L (CD154) on T cells binds CD40 on B cells, enabling class switching.
6Positive selection during T-cell maturation in the thymus ensures that surviving thymocytes:
  • A. Cannot recognize self-antigens at all
  • B. Bind with low to moderate affinity to self-MHC molecules
  • C. Express both CD4 and CD8 permanently
  • D. Undergo immediate apoptosis
Answer: B
Positive selection tests ability to recognize self-MHC.
7During negative selection in the thymus, T-cells are eliminated via apoptosis if they:
  • A. Fail to recognize self-MHC
  • B. Bind too strongly to self-peptides presented by self-MHC
  • C. Lack a T-cell receptor
  • D. Possess a functional FoxP3 gene
Answer: B
Negative selection removes high-affinity self-reactive T cells.
8The exact portion of a processed antigen that directly anchors and binds into the groove of an MHC molecule is technically called the:
  • A. Epitope
  • B. Paratope
  • C. Aggretope
  • D. Idiotype
Answer: C. Aggretope
Aggretope is the MHC-binding part of a peptide; epitope is T cell-recognized part.
9Superantigens uniquely stimulate massive, non-specific T-cell activation (and a subsequent 'cytokine storm') by:
  • A. Binding directly to the variable region of the TCR beta chain and the MHC-II alpha chain outside the peptide-binding groove
  • B. Entering the cell and bypassing the TCR completely
  • C. Multiplying inside macrophages
  • D. Destroying the thymus
Answer: A
Superantigens bypass normal antigen processing, activating many T cells.
10Which cytokine, predominantly secreted by Th2 cells, is the primary driver for B-cell class switching to IgE in allergic responses?
  • A. Interleukin-2 (IL-2)
  • B. Interleukin-4 (IL-4)
  • C. Interferon-gamma (IFN-γ)
  • D. Tumor Necrosis Factor (TNF)
Answer: B. IL-4
IL-4 induces B cells to switch to IgE production.
11Type IV (Delayed-Type) Hypersensitivity, such as a positive Tuberculin skin test, is unique among hypersensitivities because it is mediated by:
  • A. IgE
  • B. IgG and Complement
  • C. Immune complexes
  • D. T-cells and Macrophages
Answer: D
Type IV is cell-mediated, not antibody-mediated.
12In the classical pathway of the complement cascade, C1q binds specifically to the:
  • A. Variable region of IgA
  • B. Fc region of bound IgG or IgM
  • C. Hinge region of IgE
  • D. Bacterial cell wall directly
Answer: B
C1q recognizes Fc region of antibodies bound to antigen.
13The phenomenon of 'Cross-presentation' is clinically vital because it allows Dendritic cells to:
  • A. Present exogenous viral antigens on MHC-I to activate CD8+ Cytotoxic T-cells
  • B. Present endogenous antigens on MHC-II
  • C. Secrete antibodies directly
  • D. Turn into B-cells
Answer: A
Cross-presentation enables immune response against viruses that don't infect APCs.
14Natural Killer (NK) cells evaluate target cells and are inhibited from attacking healthy cells by the presence of:
  • A. High levels of MHC-II
  • B. Normal levels of MHC-I
  • C. Surface antibodies
  • D. Perforin receptors
Answer: B
NK cells are inhibited by self-MHC I via killer inhibitory receptors.
15Which process happens strictly after a B-cell has been activated by its specific antigen, leading to a progressive increase in antibody binding strength?
  • A. Somatic hypermutation (Affinity maturation)
  • B. V(D)J recombination
  • C. Negative selection
  • D. Hematopoiesis
Answer: A
Somatic hypermutation occurs in germinal centers after activation.
16The major opsonin generated by the complement cascade that rapidly coats bacterial surfaces is:
  • A. C3a
  • B. C5a
  • C. C3b
  • D. C9
Answer: C. C3b
C3b is a potent opsonin.
17Granzyme B, delivered into target cells by Cytotoxic T-cells, induces programmed cell death by directly activating:
  • A. Ribosomes
  • B. Caspases
  • C. Polymerases
  • D. Lysosomes
Answer: B. Caspases
Granzyme B activates caspases, initiating apoptosis.
18Regulatory T-cells (Tregs), crucial for maintaining peripheral tolerance and suppressing autoimmunity, are typically characterized by the expression of CD4, CD25, and the transcription factor:
  • A. T-bet
  • B. GATA3
  • C. RORγt
  • D. FOXP3
Answer: D. FOXP3
FOXP3 is the master transcription factor for Tregs.
19Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC) against IgG-coated target cells is primarily executed by which cells possessing CD16 (FcγRIII) receptors?
  • A. B-cells
  • B. Natural Killer (NK) cells
  • C. Helper T-cells
  • D. Erythrocytes
Answer: B
NK cells kill antibody-coated targets via CD16.
20Erythroblastosis fetalis (Hemolytic disease of the newborn) is a classic example of which type of hypersensitivity?
  • A. Type I
  • B. Type II
  • C. Type III
  • D. Type IV
Answer: B. Type II
Type II hypersensitivity involves antibody-mediated destruction of cells (Rh incompatibility).
21Rheumatoid factor, often found in patients with Rheumatoid Arthritis, is typically an autoantibody directed against:
  • A. Myelin basic protein
  • B. The Fc portion of host IgG
  • C. Thyroid stimulating hormone receptors
  • D. Acetylcholine receptors
Answer: B
Rheumatoid factor is an IgM anti-IgG antibody.
22The Poly-Ig receptor is responsible for the transcytosis of which molecule across epithelial barriers to mucosal surfaces?
  • A. Monomeric IgA
  • B. Dimeric IgA
  • C. Pentameric IgM
  • D. Secretory IgE
Answer: B. Dimeric IgA
Poly-Ig receptor transports dimeric IgA across epithelia.
23Adjuvants are often added to vaccines to enhance the immune response primarily by:
  • A. Acting as the main antigen
  • B. Prolonging antigen persistence and providing danger signals (co-stimulation)
  • C. Preventing antibody production
  • D. Dissolving the antigen
Answer: B
Adjuvants boost immune responses via depot effect and activation of innate immunity.
24DiGeorge syndrome results in severe immunodeficiency due to a congenital defect in the development of the 3rd and 4th pharyngeal pouches, leading to aplasia of the:
  • A. Bone Marrow
  • B. Spleen
  • C. Thymus
  • D. Lymph nodes
Answer: C. Thymus
DiGeorge syndrome involves thymic hypoplasia, causing T cell deficiency.
25The non-covalent intermolecular forces binding an antigen to an antibody include all of the following EXCEPT:
  • A. Hydrogen bonds
  • B. Van der Waals forces
  • C. Hydrophobic interactions
  • D. Disulfide (covalent) bonds
Answer: D
Antigen-antibody binding is non-covalent; disulfide bonds are within antibodies.
26Which immunoglobulin has the longest serum half-life due to its recycling via the neonatal Fc receptor (FcRn)?
  • A. IgA
  • B. IgG
  • C. IgM
  • D. IgE
Answer: B. IgG
IgG has a half-life of about 21 days due to FcRn-mediated recycling.
27Th1 Helper cells secrete IFN-γ, which is the most potent activator of:
  • A. Mast cells
  • B. Macrophages
  • C. B-cells
  • D. Eosinophils
Answer: B. Macrophages
IFN-γ activates macrophages for enhanced killing.
28Immune privilege is an evolutionary adaptation to protect vital structures from damaging inflammatory responses. Which of the following is an immune-privileged site?
  • A. Skin
  • B. Anterior chamber of the eye
  • C. Liver
  • D. Lungs
Answer: B
The eye (anterior chamber), brain, and testes are immune-privileged.
29A mutation in the Bruton's tyrosine kinase (Btk) gene blocks B-cell maturation, resulting in a disease known as X-linked Agammaglobulinemia. These patients almost entirely lack:
  • A. T-cells
  • B. Macrophages
  • C. Mature B-cells and immunoglobulins
  • D. Complement proteins
Answer: C
XLA patients have few B cells and very low antibody levels.
30Serum sickness, resulting from the injection of large amounts of foreign proteins (like horse antivenom), is characterized by the systemic deposition of antigen-antibody structures, making it a:
  • A. Type I Hypersensitivity
  • B. Type II Hypersensitivity
  • C. Type III Hypersensitivity
  • D. Type IV Hypersensitivity
Answer: C
Serum sickness is a Type III immune complex-mediated reaction.
31The very first immunoglobulin isotype synthesized and expressed on the surface of an immature B-cell in the bone marrow is:
  • A. Monomeric IgM
  • B. Dimeric IgA
  • C. Secreted IgG
  • D. IgE
Answer: A
Immature B cells express membrane IgM.
32Plasmacytoid dendritic cells play a specialized role in antiviral immunity by producing massive amounts of:
  • A. Type I Interferons (IFN-α and IFN-β)
  • B. Histamine
  • C. Interleukin-10
  • D. Membrane attack complexes
Answer: A
pDCs are professional interferon producers.
33Memory B-cells differ from naive B-cells primarily in that they:
  • A. Do not require T-cell help
  • B. Possess higher affinity BCRs and often express surface IgG, IgA, or IgE due to prior class switching
  • C. Have a shorter lifespan
  • D. Cannot undergo mitosis
Answer: B
Memory B cells have undergone isotype switching and affinity maturation.
34The 'B-cell co-receptor complex,' which significantly lowers the threshold for B-cell activation, consists of CD19, CD81, and:
  • A. CD4
  • B. CD8
  • C. CD21 (CR2)
  • D. CD28
Answer: C
CD21 (complement receptor 2) binds C3d and enhances B cell activation.
35Which interleukin serves as a major endogenous pyrogen, traveling to the hypothalamus to induce fever during an infection?
  • A. IL-1
  • B. IL-5
  • C. IL-10
  • D. IL-12
Answer: A. IL-1
IL-1, IL-6, and TNF-α are endogenous pyrogens.
36Central tolerance refers to the deletion of self-reactive lymphocytes where?
  • A. Spleen and Lymph nodes
  • B. Primary lymphoid organs (Bone marrow and Thymus)
  • C. Bloodstream
  • D. Mucosal surfaces
Answer: B
Central tolerance occurs during lymphocyte development in primary organs.
37Bare Lymphocyte Syndrome is a rare genetic disorder characterized by a complete failure to express:
  • A. Any T-cell receptors
  • B. MHC Class II molecules on APCs
  • C. Antibodies
  • D. Complement proteins
Answer: B
Bare lymphocyte syndrome type II lacks MHC II expression.
38Administration of preformed exogenous antibodies (Passive immunization) can unexpectedly inhibit the host's active immune response to a vaccine through a process called:
  • A. Epitope masking and FcγRIIB-mediated feedback inhibition
  • B. Isotype switching
  • C. Somatic hypermutation
  • D. Clonal expansion
Answer: A
Passive antibodies can suppress active immunity via feedback inhibition.
39Cyclosporine, a critical immunosuppressive drug used to prevent transplant rejection, works primarily by inhibiting calcineurin, thereby directly blocking the transcription of:
  • A. IL-2 in T-cells
  • B. Antibodies in B-cells
  • C. Histamine in mast cells
  • D. Perforin in NK cells
Answer: A
Cyclosporine inhibits IL-2 production, suppressing T cell activation.
40The three small cleavage products of the complement cascade known as 'anaphylatoxins,' which trigger mast cell degranulation and inflammation, are:
  • A. C1q, C1r, C1s
  • B. C3a, C4a, C5a
  • C. C5b, C6, C7
  • D. C7, C8, C9
Answer: B
C3a, C4a, and C5a are anaphylatoxins.
41Beta-2 microglobulin is an obligate structural component of which molecule?
  • A. MHC Class I
  • B. MHC Class II
  • C. T-Cell Receptor
  • D. Secretory component of IgA
Answer: A
β2-microglobulin associates with MHC class I heavy chain.
42The complementarity determining regions (CDRs), which dictate the ultimate specificity of an antibody, are physically located within the:
  • A. Constant domains of the heavy chain
  • B. Hinge region
  • C. Hypervariable regions of both the heavy and light chains
  • D. J-chain
Answer: C
CDRs are the hypervariable loops of variable domains.
43Activation-Induced Cytidine Deaminase (AID) is an enzyme strictly required for which two B-cell processes?
  • A. V(D)J recombination and apoptosis
  • B. Somatic hypermutation and Isotype switching
  • C. Phagocytosis and opsonization
  • D. MHC-I and MHC-II expression
Answer: B
AID initiates class switching and somatic hypermutation.
44An anamnestic response is primarily mediated by the rapid expansion of memory clones. Compared to the primary response, the lag phase of the anamnestic response is:
  • A. Much longer
  • B. Identical
  • C. Significantly shorter (often 1-3 days)
  • D. Non-existent (0 seconds)
Answer: C
Memory cells shorten the lag phase to 1-3 days.
45Which cytokine is critically required for the development, survival, and proliferation of eosinophils in defense against helminthic parasites?
  • A. IL-1
  • B. IL-2
  • C. IL-5
  • D. IL-8
Answer: C. IL-5
IL-5 is the key eosinophil growth factor.
46The Fas/Fas Ligand (FasL) pathway is heavily utilized by Cytotoxic T-cells to:
  • A. Activate B-cells
  • B. Induce apoptosis in target cells independent of the perforin/granzyme pathway
  • C. Produce antibodies
  • D. Present antigens to Helper T-cells
Answer: B
FasL on CTL binds Fas on target, triggering apoptosis.
47In the context of an allograft (transplanted organ), 'Direct Allorecognition' occurs when recipient T-cells recognize:
  • A. Processed donor peptides on recipient APCs
  • B. Intact, unprocessed donor MHC molecules on donor APCs within the graft
  • C. Recipient antibodies
  • D. Donor red blood cells
Answer: B
Direct allorecognition involves recognition of intact donor MHC.
48Wiskott-Aldrich syndrome is characterized by eczema, thrombocytopenia, and immunodeficiency due to a defect in WASp, a protein essential for:
  • A. Complement synthesis
  • B. Actin cytoskeleton reorganization in immune cells
  • C. Bone marrow hematopoiesis
  • D. V(D)J recombination
Answer: B
WASp regulates actin polymerization in hematopoietic cells.
49A toxoid, such as the tetanus toxoid, is used as a vaccine because it has lost its toxicity but fully retains its:
  • A. Enzymatic activity
  • B. Immunogenicity (epitopes)
  • C. Ability to paralyze nerves
  • D. Antibiotic resistance
Answer: B
Toxoids are inactivated toxins that retain antigenicity.
50The primary structural difference between membrane-bound IgM (the BCR) and secreted pentameric IgM lies in the:
  • A. Variable region of the light chain
  • B. Variable region of the heavy chain
  • C. Carboxy-terminal tail of the heavy chain (hydrophobic vs. hydrophilic)
  • D. Disulfide bonds in the hinge
Answer: C
Membrane IgM has a transmembrane tail; secreted IgM has a hydrophilic tail.

📄Past Paper MCQs

📅 Actual MDCAT / NUMS Questions

50 MCQs
1Which of the following immunoglobulins is the most abundant in human blood serum and can cross the placenta?
MDCAT 2019
  • A. IgA
  • B. IgE
  • C. IgG
  • D. IgM
Answer: C. IgG
IgG is the most abundant and the only Ig that crosses the placenta.
2T-lymphocytes, responsible for cell-mediated immunity, mature and differentiate in which of the following organs?
NUMS 2020
  • A. Bone marrow
  • B. Spleen
  • C. Thymus
  • D. Lymph nodes
Answer: C. Thymus
T cells mature in the thymus.
3The immunity achieved by the immediate injection of antivenom following a snake bite is an example of:
UHS MDCAT 2017
  • A. Active natural immunity
  • B. Passive natural immunity
  • C. Active artificial immunity
  • D. Passive artificial immunity
Answer: D. Passive artificial immunity
Antivenom contains preformed antibodies, giving passive artificial immunity.
4Which specific cells of the immune system are directly responsible for the massive production and secretion of antibodies?
MDCAT 2022
  • A. Helper T-cells
  • B. Cytotoxic T-cells
  • C. Plasma cells
  • D. Macrophages
Answer: C. Plasma cells
Plasma cells are antibody factories.
5The specific, localized region on the surface of an antigen that is recognized by the binding site of an antibody is called the:
NUMS 2021
  • A. Paratope
  • B. Epitope
  • C. Isotope
  • D. Clone
Answer: B. Epitope
Epitope (antigenic determinant) binds the antibody's paratope.
6Which of the following antibodies is predominantly found in bodily secretions such as tears, saliva, and maternal colostrum?
MDCAT 2018
  • A. IgD
  • B. IgE
  • C. IgG
  • D. IgA
Answer: D. IgA
Secretory IgA is present in mucosal secretions.
7The Human Immunodeficiency Virus (HIV), which causes AIDS, primarily targets and destroys which vital cells of the immune system?
PMC MDCAT 2020
  • A. CD8+ Cytotoxic T-cells
  • B. CD4+ Helper T-cells
  • C. B-lymphocytes
  • D. Natural Killer cells
Answer: B. CD4+ Helper T-cells
HIV infects CD4+ helper T cells.
8The rapid, heightened, and highly effective immune response that occurs upon a second exposure to the same pathogen is primarily due to the presence of:
NUMS 2018
  • A. Antigens
  • B. Memory cells
  • C. Neutrophils
  • D. Erythrocytes
Answer: B. Memory cells
Memory cells enable the secondary (anamnestic) response.
9Cell-mediated immunity is highly effective against all of the following EXCEPT:
MDCAT 2016
  • A. Virus-infected host cells
  • B. Cancerous cells
  • C. Free-floating extracellular bacterial toxins
  • D. Transplanted tissue cells
Answer: C
Extracellular toxins are neutralized by antibodies (humoral immunity).
10Which chemical substance is released by Cytotoxic T-cells to create pores in the cell membrane of an infected target cell?
NUMS 2019
  • A. Histamine
  • B. Perforin
  • C. Interferon
  • D. Interleukin
Answer: B. Perforin
Perforin forms pores in target cell membranes.
11The basic Y-shaped monomeric structure of an antibody molecule consists of how many polypeptide chains in total?
UHS MDCAT 2015
  • A. Two
  • B. Three
  • C. Four
  • D. Five
Answer: C. Four
Two heavy and two light chains (four total).
12Immunity acquired by a newborn baby through breast milk is classified as:
MDCAT 2021
  • A. Artificially acquired active
  • B. Naturally acquired passive
  • C. Naturally acquired active
  • D. Artificially acquired passive
Answer: B. Naturally acquired passive
Breast milk provides natural passive immunity (IgA).
13B-lymphocytes are responsible for which major branch of the adaptive immune system?
NUMS 2022
  • A. Cell-mediated immunity
  • B. Humoral immunity
  • C. Innate immunity
  • D. Non-specific immunity
Answer: B. Humoral immunity
B cells mediate humoral (antibody) immunity.
14Which class of antibodies is heavily involved in mediating Type I hypersensitivity allergic reactions and defending against parasitic worms?
MDCAT 2014
  • A. IgA
  • B. IgG
  • C. IgE
  • D. IgM
Answer: C. IgE
IgE binds to mast cells and triggers allergic responses.
15Vaccination is a prime example of inducing which type of immunity in an individual?
Sindh MDCAT 2019
  • A. Natural active
  • B. Artificial active
  • C. Natural passive
  • D. Artificial passive
Answer: B. Artificial active
Vaccines stimulate active artificial immunity.
16The region of an antibody that determines its specific class (isotype) and its mechanism of action is the:
NUMS 2017
  • A. Variable region
  • B. Hypervariable region
  • C. Constant region
  • D. Hinge region
Answer: C. Constant region
The constant region of the heavy chain determines isotype.
17What is the primary function of Helper T-cells in the immune response?
MDCAT 2020
  • A. Phagocytosis of bacteria
  • B. Direct lysis of infected cells
  • C. Secreting cytokines to activate B-cells and Cytotoxic T-cells
  • D. Producing antibodies
Answer: C
Helper T cells secrete cytokines that orchestrate immune responses.
18Major Histocompatibility Complex (MHC) proteins are critical for the immune system's ability to:
NUMS 2016
  • A. Transport oxygen
  • B. Distinguish self from non-self
  • C. Clot blood
  • D. Produce red blood cells
Answer: B
MHC presents antigens for T cell recognition, enabling self/non-self discrimination.
19Autoimmune diseases, such as Rheumatoid arthritis, occur when the immune system:
MDCAT 2013
  • A. Is compromised by a virus
  • B. Fails to recognize its own tissues and attacks them
  • C. Overreacts to harmless environmental allergens
  • D. Stops producing memory cells
Answer: B
Autoimmunity results from loss of self-tolerance.
20The process whereby antibodies bind to viral surface proteins or bacterial toxins, preventing them from interacting with host cells, is called:
UHS MDCAT 2018
  • A. Opsonization
  • B. Agglutination
  • C. Neutralization
  • D. Precipitation
Answer: C. Neutralization
Neutralization blocks pathogen or toxin activity.
21Which of the following is considered a primary lymphoid organ where lymphocyte maturation occurs?
NUMS 2020
  • A. Spleen
  • B. Thymus
  • C. Tonsils
  • D. Lymph nodes
Answer: B. Thymus
Thymus (T cells) and bone marrow (B cells) are primary lymphoid organs.
22The first class of immunoglobulins secreted into the blood during the primary immune response is typically:
MDCAT 2019
  • A. IgG
  • B. IgA
  • C. IgM
  • D. IgE
Answer: C. IgM
IgM is produced first in the primary response.
23A person who recovers from an infectious disease like measles naturally acquires:
MDCAT 2021
  • A. Active natural immunity
  • B. Passive natural immunity
  • C. Active artificial immunity
  • D. Passive artificial immunity
Answer: A. Active natural immunity
Recovery from infection provides active natural immunity.
24The specific binding site on the antibody that attaches to the epitope of an antigen is known as the:
NUMS 2015
  • A. Paratope
  • B. Hapten
  • C. Heavy chain
  • D. MHC molecule
Answer: A. Paratope
Paratope is the antigen-binding site on an antibody.
25Which of the following cellular structures acts as an Antigen Presenting Cell (APC) to Helper T-cells?
UHS MDCAT 2016
  • A. Macrophage
  • B. Erythrocyte
  • C. Neutrophil
  • D. Platelet
Answer: A. Macrophage
Macrophages are professional APCs.
26The theory that explains how a specific antigen activates only the lymphocyte bearing the matching receptor, causing it to multiply into a large population, is the:
MDCAT 2017
  • A. Clonal selection theory
  • B. Two-signal hypothesis
  • C. Sliding filament theory
  • D. Endosymbiotic theory
Answer: A
Clonal selection theory explains antigen-driven clonal expansion.
27An antigen is biologically defined as:
NUMS 2021
  • A. Any substance that causes a fever
  • B. A protein produced by B-cells
  • C. Any foreign molecule that elicits a specific immune response
  • D. A chemical that kills bacteria
Answer: C
Antigens are molecules that trigger adaptive immune responses.
28During a secondary immune response, antibody levels in the blood:
MDCAT 2022
  • A. Rise slower than in the primary response
  • B. Rise much faster and reach a higher peak
  • C. Remain identical to the primary response
  • D. Do not rise at all
Answer: B
Secondary response is faster and higher due to memory cells.
29Cytotoxic T-cells recognize foreign antigens only when they are presented on the surface of a cell in association with:
NUMS 2018
  • A. MHC Class I molecules
  • B. MHC Class II molecules
  • C. CD4 receptors
  • D. Immunoglobulin M
Answer: A. MHC Class I molecules
CD8+ T cells recognize antigen on MHC I.
30The variable regions of the light and heavy chains of an antibody are formed by the specific arrangement of:
Sindh MDCAT 2020
  • A. Carbohydrates
  • B. Amino acids
  • C. Fatty acids
  • D. Nucleotides
Answer: B. Amino acids
The variable region is composed of amino acids; sequence variation gives diversity.
31Which of the following is NOT a characteristic feature of specific/adaptive immunity?
UHS MDCAT 2019
  • A. Memory
  • B. Specificity
  • C. Immediate, generic response to any pathogen
  • D. Self-tolerance
Answer: C
Immediate, generic response is innate immunity.
32The cross-linkage and clumping together of whole cells (like bacteria or mismatched red blood cells) by antibodies is termed:
MDCAT 2018
  • A. Neutralization
  • B. Opsonization
  • C. Agglutination
  • D. Apoptosis
Answer: C. Agglutination
Agglutination clumps particulate antigens.
33The largest immunoglobulin molecule, existing as a pentamer in its secreted form, is:
NUMS 2019
  • A. IgA
  • B. IgE
  • C. IgG
  • D. IgM
Answer: D. IgM
Secreted IgM is a pentamer (900 kDa).
34Small molecules that are not immunogenic on their own but can trigger an immune response when attached to a larger carrier protein are called:
MDCAT 2015
  • A. Haptens
  • B. Epitopes
  • C. Adjuvants
  • D. Cytokines
Answer: A. Haptens
Haptens are incomplete antigens that require a carrier.
35Which subset of T-cells is responsible for turning off or dampening the immune response once an infection has been successfully cleared?
NUMS 2022
  • A. Helper T-cells
  • B. Cytotoxic T-cells
  • C. Suppressor (Regulatory) T-cells
  • D. Memory T-cells
Answer: C
Regulatory T cells suppress immune responses to maintain tolerance.
36What provides the flexibility to the Y-shaped antibody molecule, allowing it to bind to two separate antigens simultaneously?
MDCAT 2020
  • A. The J-chain
  • B. The Variable region
  • C. The Hinge region
  • D. Disulfide bridges
Answer: C. The Hinge region
The hinge region allows flexibility for bivalent binding.
37T-lymphocytes are named 'T' because their maturation is entirely dependent on the:
PMC MDCAT 2021
  • A. Thyroid gland
  • B. Tonsils
  • C. Thymus gland
  • D. Trabeculae
Answer: C. Thymus gland
T cells mature in the thymus.
38The process by which antibodies coat a pathogen, making it more easily recognized and engulfed by phagocytes, is called:
NUMS 2017
  • A. Opsonization
  • B. Lysis
  • C. Sensitization
  • D. Chemotaxis
Answer: A. Opsonization
Opsonization enhances phagocytosis.
39Which of the following is a physical barrier that belongs to innate immunity rather than specific adaptive immunity?
MDCAT 2016
  • A. Antibodies
  • B. Intact skin and mucous membranes
  • C. Helper T-cells
  • D. Plasma cells
Answer: B
Skin and mucous membranes are innate barriers.
40Interleukins are chemical messengers belonging to a broader class of signaling proteins crucial for the immune response called:
UHS MDCAT 2014
  • A. Cytokines
  • B. Hormones
  • C. Enzymes
  • D. Neurotransmitters
Answer: A. Cytokines
Interleukins are a subset of cytokines.
41Monoclonal antibodies, used heavily in diagnostics and targeted therapies, are produced in the laboratory by hybridizing a specific B-cell with a:
MDCAT 2019
  • A. Macrophage
  • B. Myeloma (cancerous) cell
  • C. T-cell
  • D. Stem cell
Answer: B. Myeloma (cancerous) cell
Hybridoma technology fuses B cells with myeloma cells.
42The transfer of IgG antibodies from a mother to her fetus across the placenta provides protection for the first few months of life. This is:
NUMS 2020
  • A. Natural active immunity
  • B. Artificial active immunity
  • C. Natural passive immunity
  • D. Artificial passive immunity
Answer: C. Natural passive immunity
Maternal IgG gives natural passive immunity.
43Which cell type does NOT express MHC Class II molecules?
MDCAT 2022
  • A. Dendritic cells
  • B. Macrophages
  • C. B-lymphocytes
  • D. Erythrocytes (Red Blood Cells)
Answer: D. Erythrocytes
Red blood cells do not express MHC class II.
44A lag phase of several days before antibodies become detectable in the blood is characteristic of:
NUMS 2016
  • A. A primary immune response
  • B. A secondary immune response
  • C. An innate immune response
  • D. An allergic reaction
Answer: A. A primary immune response
The primary response has a lag phase of 3-14 days.
45Active immunity generally lasts longer than passive immunity because active immunity strictly involves the generation of:
Sindh MDCAT 2018
  • A. Inflammatory chemicals
  • B. Immunological memory cells
  • C. Complement proteins
  • D. Exogenous antibodies
Answer: B. Immunological memory cells
Memory cells provide long-term protection.
46The specific destruction of transplanted tissue (graft rejection) is primarily mediated by:
MDCAT 2015
  • A. Red blood cells
  • B. Cell-mediated immunity (T-cells)
  • C. Humoral immunity (B-cells)
  • D. Blood platelets
Answer: B. Cell-mediated immunity (T-cells)
T cells mediate graft rejection.
47In the basic structure of an immunoglobulin molecule, the heavy and light chains are held together by:
NUMS 2019
  • A. Hydrogen bonds
  • B. Peptide bonds only
  • C. Disulfide bonds
  • D. Ionic bonds
Answer: C. Disulfide bonds
Interchain disulfide bonds connect heavy and light chains.
48Which immune cells are found in the tissues and release histamine during an allergic response, similar to the function of basophils in the blood?
UHS MDCAT 2020
  • A. Mast cells
  • B. Plasma cells
  • C. Eosinophils
  • D. Macrophages
Answer: A. Mast cells
Mast cells are tissue-resident and release histamine.
49Polio drops (OPV) given to children contain weakened viruses. This stimulates the immune system to produce which type of long-term protection?
MDCAT 2021
  • A. Active artificial immunity
  • B. Active natural immunity
  • C. Passive artificial immunity
  • D. Passive natural immunity
Answer: A. Active artificial immunity
OPV is a live attenuated vaccine inducing active artificial immunity.
50The dual nature of the specific immune system consists of the cell-mediated response and the:
NUMS 2021
  • A. Inflammatory response
  • B. Complement response
  • C. Humoral (antibody-mediated) response
  • D. Phagocytic response
Answer: C
Adaptive immunity has humoral and cell-mediated branches.

🔍Direct Search Queries — Immunity

Exact search terms students use on Google for this chapter. Click any to jump to the relevant section:

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❔Frequently Asked Questions

What is immunity?
Immunity is the ability of the body to resist and defend against pathogens (bacteria, viruses, parasites) and other foreign substances. It involves innate (non-specific) and adaptive (specific) mechanisms.
What is the difference between innate and adaptive immunity?
Innate immunity is non-specific, fast, and has no memory (skin, phagocytes, inflammation, complement). Adaptive immunity is specific, slower, and has memory (B and T lymphocytes).
What are the 4 characteristics of adaptive immunity?
Specificity (recognizes unique antigens), Diversity (vast repertoire of receptors), Memory (rapid secondary response), and Self-tolerance (avoids attacking own tissues).
What is the difference between B cells and T cells?
B cells mature in bone marrow and mediate humoral immunity (antibodies). T cells mature in thymus and mediate cell-mediated immunity. Helper T cells (CD4+) activate other immune cells; cytotoxic T cells (CD8+) kill infected cells.
What is the structure of an antibody?
An antibody is a Y-shaped protein with two heavy and two light chains. Variable regions at the tips bind antigens; constant regions determine class and function; the hinge region provides flexibility.
What are the 5 classes of immunoglobulins?
IgG (most abundant, crosses placenta), IgM (pentamer, first in primary response), IgA (secretory, mucosal immunity), IgE (allergy, parasites), IgD (B cell receptor).
What is the difference between MHC class I and MHC class II?
MHC class I is on all nucleated cells and presents endogenous antigens to CD8+ cytotoxic T cells. MHC class II is on professional APCs and presents exogenous antigens to CD4+ helper T cells.
What is the difference between humoral and cell-mediated immunity?
Humoral immunity is mediated by antibodies from B cells and targets extracellular pathogens. Cell-mediated immunity is mediated by T cells and targets intracellular pathogens, virus-infected cells, cancer cells, and transplants.
What is clonal selection?
Clonal selection is the process by which an antigen selectively activates only the lymphocytes bearing the matching receptor, causing them to proliferate into a large clone of specific effector and memory cells.
What is the difference between primary and secondary immune response?
Primary response: 3-14 day lag phase, produces IgM first then IgG, generates memory cells. Secondary response: faster (1-3 days), stronger, higher affinity IgG due to memory cells (anamnestic response).
What are the 4 types of acquired immunity?
Active natural (infection), Active artificial (vaccination), Passive natural (maternal IgG across placenta, IgA in breast milk), Passive artificial (antivenom, TIG).
What is a vaccine?
A vaccine is a biological preparation that provides active artificial immunity by stimulating the body's own immune response against a specific pathogen without causing disease. Types include live attenuated, killed, toxoid, and subunit vaccines.
What is AIDS and how does it affect the immune system?
AIDS (Acquired Immune Deficiency Syndrome) is caused by HIV, which attacks CD4+ helper T cells. Loss of helper T cells cripples both humoral and cell-mediated immunity, leading to opportunistic infections.
What is autoimmune disease?
Autoimmune disease occurs when the immune system fails to distinguish self from non-self and attacks the body's own tissues (e.g., rheumatoid arthritis, Type I diabetes, multiple sclerosis).
What are the 4 types of hypersensitivity?
Type I (IgE-mediated, allergy), Type II (antibody-mediated, cytotoxic), Type III (immune complex-mediated), Type IV (cell-mediated, delayed, T cell-mediated).
What is graft rejection?
Graft rejection is the immune response against transplanted tissue, primarily mediated by T cells recognizing foreign MHC molecules on donor tissue. Immunosuppressive drugs (cyclosporine) reduce rejection.
What is the role of memory cells?
Memory B and T cells persist after infection, providing long-term immunity. Upon re-exposure, they rapidly expand to produce a faster, stronger secondary response.
How many MCQs come from Immunity in MDCAT?
Immunity typically contributes 4–6 MCQs out of 81 Biology MCQs in MDCAT.
Which topics from Immunity are most frequently asked in MDCAT?
B vs T cells, antibody structure and classes, MHC I vs MHC II, primary vs secondary response, active vs passive immunity, vaccination, AIDS, and hypersensitivity types.
What is the difference between antigen and epitope?
An antigen is any molecule that elicits an immune response. An epitope (antigenic determinant) is the specific part of the antigen recognized and bound by an antibody or T cell receptor.

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