Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 811: A patient presents with thrombocytopenia, eczema, and recurrent infections. What is the most likely diagnosis?

A. Wiskott-Aldrich syndrome (Correct Answer)
B. DiGeorge syndrome
C. Agammaglobulinemia
D. Severe Combined Immunodeficiency (SCID)

Explanation: ### Explanation **Wiskott-Aldrich Syndrome (WAS)** is the correct diagnosis based on the classic clinical triad: **Thrombocytopenia** (presenting as petechiae or bleeding), **Eczema**, and **Recurrent infections** (due to combined B-cell and T-cell deficiency). 1. **Why it is correct:** WAS is an **X-linked recessive** disorder caused by a mutation in the *WASp* gene. This protein is essential for actin cytoskeleton remodeling in hematopoietic cells. The thrombocytopenia is unique because it features **small-sized platelets** (low mean platelet volume), which is a high-yield diagnostic marker. Patients also show low IgM levels with elevated IgA and IgE. 2. **Why the other options are incorrect:** * **DiGeorge Syndrome:** Characterized by the "CATCH-22" mnemonic (Cardiac defects, Abnormal facies, Thymic hypoplasia, Cleft palate, and Hypocalcemia). It does not typically present with thrombocytopenia or eczema. * **Agammaglobulinemia (X-linked):** Presents with recurrent pyogenic infections due to B-cell deficiency (low IgG/A/M), but lacks the associated eczema and platelet abnormalities. * **SCID:** The most severe form of immunodeficiency, presenting with failure to thrive, chronic diarrhea, and severe infections in early infancy. While it involves T and B cell defects, it does not feature the specific triad of eczema and small-platelet thrombocytopenia. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** **TIE** (Thrombocytopenia, Infections, Eczema). * **Platelet Morphology:** Small platelets (Microthrombocytes) are pathognomonic. * **Genetics:** X-linked Recessive (affects males). * **Complications:** Increased risk of autoimmune diseases and B-cell lymphomas. * **Treatment:** Hematopoietic stem cell transplant is the definitive cure.

Question 812: What are complementarity-determining regions concerned with?

A. Restricted to light chains
B. In the constant part of the immunoglobulin molecule
C. Binds to Fc receptor
D. Antigen recognition (Correct Answer)

Explanation: **Explanation:** **Complementarity-Determining Regions (CDRs)** are short, highly variable amino acid sequences located within the variable domains of both heavy and light chains of an immunoglobulin molecule. 1. **Why Option D is Correct:** CDRs are the specific sites that form the **antigen-binding pocket** (paratope). Because their structure is "complementary" to the shape of a specific epitope, they are directly responsible for **antigen recognition** and binding specificity. There are three CDRs (CDR1, CDR2, and CDR3) on each chain; among these, **CDR3** is the most variable and plays the most critical role in antigen contact. 2. **Why Other Options are Incorrect:** * **Option A:** CDRs are found in **both** heavy (VH) and light (VL) chains, not restricted to light chains. * **Option B:** They are located in the **Variable (V) regions**, not the constant (C) regions. The constant regions determine the biological properties (isotype) of the antibody, not its binding specificity. * **Option C:** The **Fc portion** (formed by the constant regions of heavy chains) binds to Fc receptors on immune cells. CDRs are part of the **Fab portion**. **High-Yield Clinical Pearls for NEET-PG:** * **Hypervariable Regions:** CDRs are synonymous with hypervariable regions. * **Framework Regions (FRs):** These are the stable regions between CDRs that provide the structural scaffold for the variable domain. * **Diversity:** The immense diversity of CDRs is generated by V(D)J recombination and somatic hypermutation. * **Idiotype:** The sum total of the CDRs determines the "Idiotype" of an immunoglobulin molecule.

Question 813: What is the C3 convertase in the alternative complement pathway?

A. C4b2b
B. C3b
C. C3bBb (Correct Answer)
D. C3a

Explanation: ### Explanation The complement system is a crucial component of innate immunity, consisting of a cascade of proteins that lead to opsonization, inflammation, and cell lysis. **Correct Answer: C. C3bBb** In the **Alternative Pathway**, activation begins with the spontaneous hydrolysis of C3 ("tick-over"). When C3b binds to a pathogen surface, it recruits **Factor B**. Factor B is then cleaved by **Factor D** into Ba and Bb. The resulting complex, **C3bBb**, acts as the **C3 convertase**, which further cleaves more C3 into C3a and C3b, creating a powerful amplification loop. **Analysis of Incorrect Options:** * **A. C4b2b:** This is the C3 convertase for the **Classical** and **Lectin** pathways. It is formed after C1 (Classical) or MASP (Lectin) cleaves C4 and C2. * **B. C3b:** While C3b is a component of the convertase and acts as a potent **opsonin**, it lacks enzymatic activity on its own to cleave C3. * **C. C3a:** This is a small peptide fragment released after C3 cleavage. It functions as an **anaphylatoxin**, triggering mast cell degranulation and inflammation. **High-Yield Facts for NEET-PG:** * **Properdin (Factor P):** This is the only known positive regulator of complement; it stabilizes the C3bBb complex, increasing its half-life. * **C5 Convertase:** In the alternative pathway, adding another C3b to the C3 convertase forms **C3bBb3b** (the C5 convertase). * **Membrane Attack Complex (MAC):** All pathways converge at the formation of the MAC (**C5b-C9**). * **Deficiency:** Deficiency of early alternative pathway components (e.g., Properdin, Factor D) increases susceptibility to *Neisseria* infections.

Question 814: Which one of the following antibodies is produced rapidly and in high amounts during a secondary immune response?

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

Explanation: ### Explanation The correct answer is **IgG**. **1. Why IgG is correct:** The secondary immune response (anamnestic response) occurs when the immune system encounters an antigen for the second or subsequent time. This response is mediated by **Memory B-cells**. Upon re-exposure, these cells rapidly proliferate and undergo **class switching**, leading to the production of high titers of **IgG**. Compared to the primary response, the secondary response has a shorter lag phase, a faster rate of antibody rise, and reaches a much higher peak concentration. Furthermore, the antibodies produced have a higher affinity for the antigen (affinity maturation). **2. Why the other options are incorrect:** * **IgM:** This is the first antibody produced during the **primary immune response**. In a secondary response, IgM is produced in low amounts or remains similar to primary levels, while IgG dominates. * **IgA:** This is the primary secretory antibody found in mucosal surfaces (tears, saliva, colostrum). While it can be part of a secondary response in mucosal immunity, IgG is the systemic hallmark of secondary humoral immunity. * **IgM and IgG:** While both are present, they are not produced in equal "high amounts" rapidly. The defining characteristic of the secondary response is the massive shift toward IgG. **3. NEET-PG High-Yield Pearls:** * **Primary Response:** Long lag phase (5–10 days), predominantly **IgM**, lower antibody titer. * **Secondary Response:** Short lag phase (1–3 days), predominantly **IgG**, much higher titer, persists longer. * **Affinity Maturation:** This process occurs during the secondary response, making the IgG "fit" the antigen better than the initial IgM. * **Placental Transfer:** IgG is the only antibody that crosses the placenta, providing passive immunity to the fetus.

Question 815: What is the first chemical barrier encountered by microorganisms at common exposed sites?

A. Lysozyme
B. Acidic pH (Correct Answer)
C. Skin
D. Lactose

Explanation: **Explanation:** The body’s first line of defense consists of physical and chemical barriers. Among these, **Acidic pH** is the most ubiquitous and immediate chemical barrier encountered by microorganisms at major portals of entry. 1. **Why Acidic pH is correct:** Most pathogenic bacteria are neutrophiles and cannot survive in highly acidic environments. The stomach maintains a very low pH (1.0–3.0) due to hydrochloric acid, which sterilizes ingested food. Similarly, the skin has an "acid mantle" (pH 4.5–5.5) created by lactic acid and fatty acids in sweat and sebum, and the adult vagina maintains an acidic pH (4.0–4.5) due to *Lactobacillus* activity, preventing the overgrowth of pathogens like *Candida*. 2. **Analysis of Incorrect Options:** * **Lysozyme (A):** While an important antibacterial enzyme found in tears and saliva, it is specific to degrading peptidoglycan in Gram-positive bacteria. It is not as universally distributed or as foundational as pH. * **Skin (C):** This is a **physical/mechanical barrier**, not a chemical one. It acts as a structural wall (keratinized epithelium) rather than a biochemical deterrent. * **Lactose (D):** This is a disaccharide sugar found in milk; it does not function as a primary chemical barrier against infection. **High-Yield NEET-PG Pearls:** * **Lysozyme:** Cleaves the β-1,4 glycosidic bond between NAM and NAG. * **Defensins:** Cationic peptides that create pores in bacterial membranes; found in the GI and lower respiratory tracts. * **Vaginal pH:** In pre-pubertal and post-menopausal females, the pH is neutral/alkaline, making them more susceptible to infections compared to women of reproductive age.

Question 816: Which of the following is a superantigen?

A. Exfoliative toxin of Staphylococcus (Correct Answer)
B. Lipopolysaccharide of Gram-negative bacteria
C. Enterotoxin of V. cholerae
D. Shiga toxin of EHEC

Explanation: ### Explanation **Correct Answer: A. Exfoliative toxin of Staphylococcus** **Understanding Superantigens:** Superantigens (SAgs) are potent immunostimulatory molecules that bypass the standard antigen-processing pathway. Unlike conventional antigens, which are processed and presented in the MHC-II groove, superantigens bind **directly and non-specifically** to the **outer surface of the MHC-II molecule** on Antigen Presenting Cells (APCs) and the **Vβ region of the T-cell receptor (TCR)**. This results in the massive activation of up to 20% of the body’s T-cells, leading to a "cytokine storm" (IFN-γ, IL-1, IL-6, TNF-α). **Staphylococcal Exfoliative toxins (ETA, ETB)**, responsible for Staphylococcal Scalded Skin Syndrome (SSSS), function as superantigens. **Analysis of Incorrect Options:** * **B. Lipopolysaccharide (LPS):** LPS is a classic **Endotoxin** found in the outer membrane of Gram-negative bacteria. It activates the innate immune system via TLR-4 but does not act as a T-cell superantigen. * **C. Enterotoxin of V. cholerae:** This is an **A-B subunit toxin** that acts by ADP-ribosylation of Gs proteins, leading to increased cAMP and secretory diarrhea. It is not a superantigen. * **D. Shiga toxin of EHEC:** This is also an **A-B toxin** that inhibits protein synthesis by damaging the 28S ribosomal RNA. It is associated with Hemolytic Uremic Syndrome (HUS). **High-Yield Clinical Pearls for NEET-PG:** * **Common Superantigens:** * *Staphylococcus aureus:* TSST-1 (Toxic Shock Syndrome), Enterotoxins (Food poisoning), Exfoliative toxin. * *Streptococcus pyogenes:* Erythrogenic toxin (SpeA and SpeC). * **Key Feature:** They do **not** require processing by APCs and bind to the **Vβ region** of TCR. * **Clinical Result:** Massive release of **TNF-α and IL-1**, leading to shock and multi-organ failure.

Question 817: Which of the following statements is true about interferon?

A. It is a synthetic antiviral agent.
B. It inhibits viral replication in cells. (Correct Answer)
C. It is specific for a particular virus.
D. None of the above.

Explanation: ### Explanation **Correct Option: B. It inhibits viral replication in cells.** Interferons (IFNs) are natural glycoproteins produced by host cells in response to viral infections. They do not kill viruses directly; instead, they act as signaling molecules. When a cell is infected, it secretes IFNs which bind to receptors on neighboring uninfected cells. This triggers the synthesis of **antiviral proteins (AVPs)**, such as *2',5'-oligoadenylate synthetase* and *protein kinase R*, which degrade viral mRNA and inhibit protein synthesis, thereby preventing viral replication. **Analysis of Incorrect Options:** * **Option A:** Interferons are **endogenous** biological products (cytokines) produced by the body, not synthetic drugs, although recombinant versions are used therapeutically. * **Option C:** Interferons are **host-specific but not virus-specific**. This means human interferon will work against any virus (Influenza, HBV, etc.) in a human cell, but human interferon is generally ineffective in other species (e.g., mice). **High-Yield NEET-PG Pearls:** * **Classification:** * **Type I (IFN-α, IFN-β):** Primarily antiviral and produced by leucocytes and fibroblasts. * **Type II (IFN-γ):** Produced by Th1 cells/NK cells; primarily an **immunomodulator** (activates macrophages). * **Mechanism:** They induce an "antiviral state" in surrounding cells. * **Clinical Use:** IFN-α is used in the treatment of Chronic Hepatitis B, Hepatitis C, and Kaposi Sarcoma. * **Key Fact:** Interferons are the first line of innate immune defense against viral infections before antibodies appear.

Question 818: Which of the following cytokines is associated with the development of cell-mediated immunity?

A. IL-3
B. IL-4
C. IL-5
D. IL-12 (Correct Answer)

Explanation: ### Explanation **Correct Option: D (IL-12)** Cell-mediated immunity (CMI) is primarily driven by the **Th1 subset** of T-helper cells. The differentiation of naive T-cells (Th0) into Th1 cells is triggered by **Interleukin-12 (IL-12)**, which is secreted by macrophages and dendritic cells in response to intracellular pathogens. IL-12 stimulates Th1 cells to produce **IFN-γ**, which activates macrophages and enhances the killing of intracellular organisms (e.g., *M. tuberculosis*). **Analysis of Incorrect Options:** * **IL-3 (Option A):** This is a hematopoietic growth factor that stimulates the proliferation and differentiation of myeloid progenitor cells in the bone marrow. It is not a primary driver of CMI. * **IL-4 (Option B):** This is the signature cytokine for **Humoral Immunity**. It promotes the differentiation of Th0 cells into **Th2 cells**, which facilitate B-cell antibody class switching to IgE. * **IL-5 (Option C):** Also produced by Th2 cells, IL-5 is primarily responsible for the activation and chemotaxis of **eosinophils** and stimulates B-cells to produce IgA. **High-Yield NEET-PG Pearls:** * **The "Th1 vs. Th2" Paradigm:** * **Th1 (CMI):** Induced by **IL-12** and IFN-γ. Key cytokines: **IL-2, IFN-γ, TNF-β**. * **Th2 (Humoral):** Induced by **IL-4**. Key cytokines: **IL-4, IL-5, IL-6, IL-10, IL-13**. * **IL-12 Deficiency:** Patients with IL-12 receptor mutations are highly susceptible to mycobacterial infections due to impaired Th1 response. * **Cross-regulation:** IFN-γ (Th1) inhibits Th2 proliferation, while IL-10 (Th2) inhibits Th1 cytokine production.

Question 819: A patient has an increased antibody titer to delta agent. What would you most likely suspect?

A. Fifth disease
B. Susceptibility to chickenpox
C. Possible subacute sclerosing panencephalitis (SSPE)
D. Possible hepatitis B infection (Correct Answer)

Explanation: **Explanation:** The **Delta agent** refers to the **Hepatitis D Virus (HDV)**. HDV is a "defective" RNA virus that requires the presence of the **Hepatitis B Virus (HBV)** to replicate. Specifically, HDV uses the Hepatitis B surface antigen (HBsAg) as its outer envelope to package its genome and infect other hepatocytes. Therefore, a positive antibody titer to the delta agent is a definitive marker for a co-infection or super-infection with **Hepatitis B**. **Analysis of Options:** * **Option A (Fifth disease):** Caused by **Parvovirus B19**. It typically presents with a "slapped-cheek" rash in children and is unrelated to the delta agent. * **Option B (Susceptibility to chickenpox):** Chickenpox is caused by the **Varicella-Zoster Virus (VZV)**. Susceptibility is determined by the absence of anti-VZV IgG antibodies, not delta agent titers. * **Option C (SSPE):** Subacute sclerosing panencephalitis is a rare, progressive neurological complication caused by a persistent **Measles virus** infection. Diagnosis involves high titers of measles antibodies in the CSF and serum. **High-Yield Clinical Pearls for NEET-PG:** * **Co-infection:** Simultaneous infection of HBV and HDV. It usually results in acute hepatitis but has a low risk of chronicity. * **Super-infection:** HDV infection in a chronic HBV carrier. This carries a much higher risk of fulminant hepatitis and rapid progression to cirrhosis. * **Prevention:** The Hepatitis B vaccine is protective against HDV because HDV cannot exist without HBV. * **Genome:** HDV has a circular, single-stranded negative-sense RNA genome.

Question 820: Which of the following cells present MHC Class II molecules?

A. Macrophage
B. Dendritic cells
C. Lymphocyte
D. All of the above (Correct Answer)

Explanation: **Explanation:** The core concept tested here is the distribution of **Major Histocompatibility Complex (MHC)** molecules. While MHC Class I is expressed on almost all nucleated cells, **MHC Class II** expression is restricted to **Professional Antigen-Presenting Cells (pAPCs)**. 1. **Dendritic Cells (Option B):** These are the most potent pAPCs. They are the only cells capable of activating naive T-cells, making them the primary initiators of the adaptive immune response. 2. **Macrophages (Option A):** These act as pAPCs by phagocytosing pathogens and presenting exogenous antigens via MHC II to Helper T-cells (CD4+), triggering cytokine release. 3. **Lymphocytes (Option C):** Specifically, **B-lymphocytes** are professional APCs. They internalize antigens via surface immunoglobulins and present them via MHC II to T-cells to receive "help" for antibody production. (Note: While T-cells are also lymphocytes, in the context of this standard MCQ, "lymphocytes" refers to the B-cell population's APC function). Since all three cell types are professional APCs, **Option D** is the correct answer. **High-Yield NEET-PG Pearls:** * **MHC Class II** interacts exclusively with **CD4+ T-helper cells** (Rule of 8: 2 × 4 = 8). * **MHC Class I** interacts with **CD8+ Cytotoxic T-cells** (1 × 8 = 8). * **Exceptions:** While MHC II is usually on pAPCs, "non-professional" APCs (like vascular endothelial cells or thymic epithelial cells) can express MHC II under the influence of **Interferon-gamma (IFN-γ)**. * **Human Leukocyte Antigens (HLA):** MHC II is encoded by the **HLA-DP, DQ, and DR** loci.

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Immunology — MCQs

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