Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 221: The membrane attack complex is formed by which of the following complement components?

A. C3 (Correct Answer)
B. C5
C. C7
D. C9

Explanation: **Explanation:** The **Membrane Attack Complex (MAC)**, also known as the terminal complement complex, is the final effector of the complement cascade. It is responsible for creating pores in the lipid bilayer of target cells, leading to osmotic lysis. **Why Option A (C3) is the correct answer:** While the MAC is physically composed of C5b through C9, the question asks which component is involved in its **formation**. **C3** is the central and most abundant component of the complement system. The formation of the MAC is strictly dependent on the cleavage of C3 into C3b. C3b then joins the C3 convertase to form **C5 convertase** (C4b2a3b or C3bBb3b). Without the activation and contribution of C3, the cascade cannot progress to the terminal pathway where the MAC is assembled. **Analysis of Incorrect Options:** * **B, C, and D (C5, C7, C9):** These are structural components of the MAC itself. C5b initiates the assembly, C7 anchors the complex to the membrane, and C9 polymerizes to form the actual pore. While they are part of the complex, the entire process is initiated and driven by the pivotal cleavage of **C3**. **High-Yield Clinical Pearls for NEET-PG:** * **C3 Deficiency:** The most severe complement deficiency; it leads to recurrent pyogenic infections and Type III hypersensitivity reactions. * **MAC Deficiency (C5-C9):** Specifically predisposes individuals to recurrent **Neisseria** infections (meningitis and gonorrhea). * **CH50 Assay:** Used to screen for deficiencies in the classical and terminal complement pathways. * **Paroxysmal Nocturnal Hemoglobinuria (PNH):** Caused by a deficiency in DAF (CD55) and MIRL (CD59), which normally protect host cells from MAC-mediated lysis.

Question 222: To which cell does IgE bind?

A. T cells
B. B cells
C. Mast cells (Correct Answer)
D. NK cells

Explanation: **Explanation:** The correct answer is **Mast cells**. **Why Mast Cells?** IgE (Immunoglobulin E) is the primary mediator of Type I hypersensitivity reactions. Mast cells and basophils possess high-affinity surface receptors known as **FcεRI**. The Fc portion of the IgE molecule binds to these receptors with extreme tenacity. When an allergen later cross-links these bound IgE molecules, it triggers mast cell degranulation, releasing inflammatory mediators like histamine, leukotrienes, and prostaglandins. **Analysis of Incorrect Options:** * **T cells:** These cells utilize T-cell receptors (TCR) to recognize antigens presented by MHC molecules. They do not possess high-affinity receptors for IgE. * **B cells:** While B cells produce IgE (after class-switching), they primarily express IgM and IgD as membrane-bound receptors. They do not typically bind circulating IgE via FcεRI. * **NK cells:** Natural Killer cells possess **FcγRIII (CD16)**, which binds to **IgG**, facilitating Antibody-Dependent Cellular Cytotoxicity (ADCC), but they do not bind IgE. **High-Yield Clinical Pearls for NEET-PG:** * **Receptor Specificity:** **FcεRI** is the high-affinity receptor (found on Mast cells/Basophils); **FcεRII (CD23)** is the low-affinity receptor (found on B cells and macrophages). * **Prausnitz-Küstner (PK) Reaction:** A classic experiment demonstrating that the "reaginic antibody" (IgE) in serum can sensitize the skin of a non-allergic individual by binding to local mast cells. * **Parasitic Infections:** IgE also plays a role in defense against helminths by coating the parasite; eosinophils then bind to the IgE via Fc receptors to release major basic protein (ADCC).

Question 223: Which of the following conditions is caused by deficiency of complement C3 and C3b?

A. Hereditary angioneurotic edema
B. Systemic lupus erythematosus (SLE)
C. Severe recurrent pyogenic infection (Correct Answer)
D. Collagen vascular disease

Explanation: **Explanation:** **Core Concept:** Complement **C3** is the central hub of the complement system, where the classical, lectin, and alternative pathways converge. Its primary functions include **opsonization** (via C3b) and the initiation of the **Membrane Attack Complex (MAC)**. **Why C is correct:** C3b acts as a major opsonin, coating bacteria to facilitate phagocytosis by neutrophils and macrophages. A deficiency in C3 or C3b leads to a failure in opsonization and an inability to clear encapsulated bacteria (e.g., *Streptococcus pneumoniae*, *Haemophilus influenzae*). This results in **severe, recurrent pyogenic (pus-forming) infections**, typically starting early in life. **Why other options are incorrect:** * **A. Hereditary Angioneurotic Edema:** This is caused by a deficiency of **C1 esterase inhibitor**, leading to overproduction of bradykinin. * **B & D. SLE and Collagen Vascular Diseases:** Deficiencies in **early components** of the classical pathway (**C1, C4, and C2**) are strongly associated with SLE-like syndromes. This is because these components are essential for the clearance of immune complexes; their absence leads to tissue deposition and inflammation. **High-Yield Clinical Pearls for NEET-PG:** * **C1, C2, C4 deficiency:** Associated with Immune complex diseases (SLE). **C2** is the most common complement deficiency. * **C3 deficiency:** Most severe; leads to recurrent pyogenic infections and Type III hypersensitivity. * **C5–C9 (MAC) deficiency:** Specifically predisposes to recurrent **Neisserial infections** (Meningitis/Gonorrhea). * **CH50 Assay:** Used to screen for classical pathway integrity; **AH50** screens the alternative pathway.

Question 224: Streptococcal cell wall polysaccharide cross-reacts with which of the following?

A. Myocardial muscle (Correct Answer)
B. Cardiac valve
C. Endocardium
D. Synovial fluid

Explanation: This question tests your knowledge of **Molecular Mimicry** in the pathogenesis of Acute Rheumatic Fever (ARF) following a *Streptococcus pyogenes* (Group A Strep) infection. ### **Explanation** The core mechanism of ARF is a Type II hypersensitivity reaction where antibodies produced against streptococcal antigens cross-react with human host tissues due to structural similarities. * **The Correct Answer (A):** The **Group A Streptococcal cell wall polysaccharide** (specifically the C-carbohydrate) contains N-acetylglucosamine. This epitope structurally mimics the glycoproteins found in the **myocardial muscle** (specifically the sarcolemma) and the **cardiac valves**. However, in standard microbiological teaching and classic NEET-PG patterns, the cell wall polysaccharide is most specifically linked to the **myocardium**. ### **Analysis of Other Options** * **B & C (Cardiac Valve/Endocardium):** While valves are damaged in ARF, the cross-reactivity here is primarily attributed to the **Streptococcal M-protein** and **Hyaluronic acid capsule**, rather than the cell wall polysaccharide. * **D (Synovial Fluid):** Arthritis in ARF is inflammatory but not typically due to direct polysaccharide cross-reactivity. The **Hyaluronic acid capsule** of the bacteria mimics the hyaluronic acid in human joints, leading to the "migratory polyarthritis" seen clinically. ### **High-Yield Clinical Pearls for NEET-PG** To master this topic, remember these specific antigen-tissue pairings: 1. **M-Protein:** Cross-reacts with **Myosin** (Myocardium) and Sarcolemma. 2. **Cell Wall Polysaccharide (C-Substance):** Cross-reacts with **Myocardial Sarcolemma** and Glycoproteins of heart valves. 3. **Capsule (Hyaluronic Acid):** Cross-reacts with **Synovium/Joints**. 4. **Cytoplasmic Membrane:** Cross-reacts with the **Subthalamic and Caudate nuclei** (leading to Sydenham’s Chorea). 5. **Protoplast Membrane:** Cross-reacts with the **Glomerular basement membrane** (Post-Streptococcal Glomerulonephritis).

Question 225: Which immunoglobulin is primarily involved in Type I hypersensitivity reactions (immediate hypersensitivity)?

A. IgG
B. IgA
C. IgE (Correct Answer)
D. IgM

Explanation: **Explanation:** **Correct Answer: C. IgE** Type I hypersensitivity (Immediate Hypersensitivity) is mediated by **IgE antibodies**. Upon initial exposure to an allergen, B-cells undergo class switching to produce IgE, which binds to high-affinity receptors (**FcεRI**) on the surface of **mast cells and basophils** (Sensitization). Upon re-exposure, the allergen crosses-links the membrane-bound IgE, triggering degranulation and the release of pharmacological mediators like **histamine**, leukotrienes, and prostaglandins. This leads to clinical manifestations such as anaphylaxis, asthma, and urticaria. **Why other options are incorrect:** * **IgG:** Primarily involved in **Type II** (cytotoxic) and **Type III** (immune-complex) hypersensitivity. It is the most abundant antibody in serum and crosses the placenta. * **IgA:** The primary secretory immunoglobulin found in colostrum, saliva, and mucosal surfaces. It provides local immunity but does not mediate Type I reactions. * **IgM:** The first antibody produced in a primary immune response and the most efficient at activating the classical complement pathway. It is involved in Type II hypersensitivity (e.g., ABO incompatibility). **High-Yield NEET-PG Pearls:** * **Prausnitz-Küstner (PK) Reaction:** A classic test used to demonstrate IgE-mediated passive transfer of hypersensitivity. * **Atopy:** A genetic predisposition to produce excessive IgE in response to common environmental allergens. * **Eosinophilia:** Often accompanies Type I reactions due to the release of Eosinophil Chemotactic Factor of Anaphylaxis (ECF-A). * **Drug of Choice:** Epinephrine (1:1000 IM) is the gold standard treatment for systemic Type I anaphylactic reactions.

Question 226: Which of the following interleukins is characteristically produced in a TH1 response?

A. IL-2 (Correct Answer)
B. IL-4
C. IL-5
D. IL-10

Explanation: ### Explanation The differentiation of **CD4+ T-helper cells** into specific subsets (Th1, Th2, Th17) is a fundamental concept in immunology. The choice of subset depends on the cytokine environment and determines the type of immune response generated. **Why Option A (IL-2) is Correct:** The **Th1 response** is primarily responsible for **cell-mediated immunity** and defense against intracellular pathogens (e.g., *M. tuberculosis*). Th1 cells characteristically produce **IL-2**, **IFN-γ** (Interferon-gamma), and **TNF-β**. * **IL-2** acts as a potent T-cell growth factor, promoting the clonal expansion of T-cells. * **IFN-γ** activates macrophages and stimulates B-cells to produce IgG (opsonizing antibodies). **Why the Other Options are Incorrect:** Options B, C, and D are characteristic of a **Th2 response**, which mediates **humoral immunity** and defense against helminths/allergens: * **IL-4:** Induces B-cell class switching to **IgE** and promotes further Th2 differentiation. * **IL-5:** Responsible for **eosinophil** activation and recruitment. * **IL-10:** An **anti-inflammatory** cytokine that inhibits Th1 responses by suppressing IL-12 production from macrophages. **High-Yield Clinical Pearls for NEET-PG:** * **Master Switch:** The transcription factor for Th1 is **T-bet**, while for Th2 it is **GATA-3**. * **Inducing Cytokine:** **IL-12** (from macrophages/DCs) is the primary driver for Th1 differentiation. * **Leprosy Link:** A strong **Th1 response** leads to Tuberculoid leprosy (contained), while a dominant **Th2 response** leads to Lepromatous leprosy (disseminated). * **Th17:** Produces **IL-17**; involved in fungal infections and autoimmune diseases.

Question 227: Which of the following is an example of type-III hypersensitivity?

A. Contact dermatitis
B. Hemolytic anemia
C. Serum sickness (Correct Answer)
D. Good pasture syndrome

Explanation: **Explanation:** **Type III Hypersensitivity** is an **immune-complex-mediated** reaction. It occurs when soluble antigen-antibody complexes (usually IgG or IgM) are not adequately cleared by the reticuloendothelial system. these complexes deposit in tissues (like blood vessel walls, joints, or kidneys), activate the complement system, and recruit neutrophils, leading to tissue damage and vasculitis. * **Serum Sickness (Option C):** This is the classic systemic example of Type III hypersensitivity. It occurs after the administration of foreign serum or certain drugs (e.g., penicillin). Symptoms typically appear 7–14 days later and include fever, rash, polyarthritis, and glomerulonephritis due to widespread immune complex deposition. **Analysis of Incorrect Options:** * **A. Contact Dermatitis:** This is a **Type IV (Delayed-type)** hypersensitivity reaction mediated by T-cells (CD4+ and CD8+), not antibodies. * **B. Hemolytic Anemia:** This is a **Type II (Cytotoxic)** hypersensitivity reaction where antibodies (IgG/IgM) bind directly to antigens on the surface of red blood cells, leading to their destruction. * **D. Goodpasture Syndrome:** This is also a **Type II** reaction. It involves anti-GBM antibodies binding to the basement membranes of the lungs and kidneys (fixed antigens). **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Hypersensitivity (ACID):** **A**naphylactic (I), **C**ytotoxic (II), **I**mmune-Complex (III), **D**elayed (IV). * **Type III Examples:** Systemic Lupus Erythematosus (SLE), Post-streptococcal glomerulonephritis (PSGN), Arthus reaction, and Farmer’s Lung. * **Key Difference:** In Type II, antibodies bind to **fixed cell-surface antigens**; in Type III, antibodies bind to **soluble antigens** in the circulation.

Question 228: Which of the following best describes the mechanism(s) of action of CTLA-4?

A. Signal 1 and costimulation
B. Competition and inhibition (Correct Answer)
C. Activation and proliferation
D. ZAP-70 and ITAMs

Explanation: **Explanation:** **CTLA-4 (Cytotoxic T-Lymphocyte Associated Protein 4)** is a critical checkpoint molecule that functions as a negative regulator of T-cell activation. Its mechanism of action is twofold: 1. **Competition:** CTLA-4 has a significantly higher affinity and avidity for **B7 ligands (CD80/CD86)** on Antigen-Presenting Cells (APCs) than the stimulatory receptor **CD28**. By outcompeting CD28 for binding, it prevents the necessary "Signal 2" (costimulation) required for T-cell activation. 2. **Inhibition:** Upon binding, CTLA-4 delivers inhibitory signals to the T-cell and can actively remove B7 molecules from the APC surface via trans-endocytosis, leading to functional suppression. **Analysis of Incorrect Options:** * **Option A:** Signal 1 (MHC-TCR) and costimulation (CD28-B7) are the requirements for T-cell **activation**, whereas CTLA-4 acts to terminate this process. * **Option C:** CTLA-4 leads to **anergy** or cell cycle arrest, the exact opposite of activation and proliferation. * **Option D:** ZAP-70 and ITAMs are components of the **stimulatory** TCR signaling pathway. CTLA-4 signaling involves phosphatases (like SHP-2) that counteract these stimulatory kinases. **High-Yield Clinical Pearls for NEET-PG:** * **Abatacept:** A fusion protein (CTLA-4 + IgG1 Fc) used in Rheumatoid Arthritis to block costimulation. * **Ipilimumab:** A monoclonal antibody against CTLA-4 used in **Melanoma**; it "removes the brakes" on the immune system to enhance anti-tumor responses. * **Location:** While CD28 is constitutively expressed, CTLA-4 is upregulated *after* T-cell activation (primarily in secondary lymphoid organs).

Question 229: What is a common consequence of hypogammaglobulinemia?

A. Chronic recurrent sinusitis (Correct Answer)
B. Epistaxis
C. Contractures
D. Eczema

Explanation: **Explanation:** **Hypogammaglobulinemia** is a state of immune deficiency characterized by low levels of serum antibodies (Immunoglobulins). Antibodies, particularly **IgA and IgG**, are the primary defense against encapsulated bacteria at mucosal surfaces. 1. **Why Option A is Correct:** Antibodies are essential for the opsonization and neutralization of pathogens. In hypogammaglobulinemia (as seen in Common Variable Immunodeficiency or X-linked Agammaglobulinemia), the body cannot effectively clear bacteria from the respiratory tract. This leads to **chronic recurrent sinopulmonary infections**, such as sinusitis, otitis media, and pneumonia, typically caused by *Streptococcus pneumoniae* and *Haemophilus influenzae*. 2. **Why Other Options are Incorrect:** * **B. Epistaxis:** This is usually related to local trauma, coagulopathies, or platelet disorders, not antibody deficiency. * **C. Contractures:** These are permanent shortenings of muscles or joints, often seen in neurological disorders or severe burns, and have no immunological basis. * **D. Eczema:** While eczema is associated with some immunodeficiencies (like **Wiskott-Aldrich Syndrome** or **Hyper-IgE Syndrome/Job Syndrome**), it is not a direct consequence of generalized hypogammaglobulinemia itself. **NEET-PG High-Yield Pearls:** * **X-linked Agammaglobulinemia (Bruton’s):** Characterized by a defect in **BTK (Bruton Tyrosine Kinase)**, leading to a failure of B-cell maturation. Patients present with recurrent infections after 6 months of age (once maternal IgG wanes). * **Common Variable Immunodeficiency (CVID):** Presents later in life (2nd–3rd decade) with low IgG/IgA/IgM and an increased risk of autoimmune diseases and lymphoma. * **Selective IgA Deficiency:** The most common primary immunodeficiency; often asymptomatic but can cause respiratory/GI infections and **anaphylaxis during blood transfusions** due to anti-IgA antibodies.

Question 230: T-cells are identified by which of the following methods?

A. Rosette formation with sheep red blood cells (Correct Answer)
B. Immunoglobulins on their surface
C. EAC rosette with sheep erythrocytes
D. Filamentous projections on their surface

Explanation: **Explanation:** T-cells are identified by their ability to form **E-rosettes** (Erythrocyte rosettes) when incubated with sheep red blood cells (SRBCs). This occurs because T-cells possess the **CD2 receptor** (LFA-2), which has a natural affinity for the LFA-3 (CD58) homologue found on sheep erythrocytes. When mixed, the SRBCs surround the T-cell, creating a cluster that resembles a rose under the microscope. **Analysis of Options:** * **Option A (Correct):** Rosette formation with SRBCs is a classic laboratory marker for T-lymphocytes. * **Option B (Incorrect):** Surface Immunoglobulins (sIg), specifically IgM and IgD, are the hallmark markers for **B-cells**, not T-cells. * **Option C (Incorrect):** **EAC rosettes** (Erythrocyte-Amboceptor-Complement) involve SRBCs coated with antibody and complement. These bind to CR1/CR2 receptors found on **B-cells** and Macrophages. * **Option D (Incorrect):** Filamentous or "hairy" projections are characteristic of **Hairy Cell Leukemia** (a B-cell neoplasm), not a general identification method for T-cells. **High-Yield Clinical Pearls for NEET-PG:** * **CD3** is the most specific definitive marker for all T-cells (part of the TCR complex). * **CD4** and **CD8** are used to further sub-classify T-cells into Helper and Cytotoxic types, respectively. * **Flow Cytometry** is the modern "gold standard" for identifying and counting T-cell subsets (e.g., CD4 counts in HIV). * **Pan-T cell markers:** CD2, CD3, CD5, and CD7.

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Immunology — MCQs

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