Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 781: Which of the following are pro-inflammatory cytokines?

A. IL-1, IL-2, IL-4
B. IL-4, IL-2, IL-1
C. IL-1, IL-6, IL-8 (Correct Answer)
D. IL-2, IL-1, IL-4

Explanation: ### Explanation **Core Concept: Pro-inflammatory Cytokines** Pro-inflammatory cytokines are signaling molecules produced predominantly by activated macrophages and T-cells. They initiate and amplify the inflammatory response by promoting leukocyte recruitment, increasing vascular permeability, and inducing the acute-phase response. The "classic" triad of systemic pro-inflammatory cytokines includes **IL-1, IL-6, and TNF-α**. **Why Option C is Correct:** * **IL-1:** A key mediator of the host inflammatory response; it induces fever (endogenous pyrogen) and activates vascular endothelium. * **IL-6:** The primary inducer of **Acute Phase Reactants** (like CRP) from the liver and stimulates B-cell differentiation. * **IL-8:** A potent **chemotactic factor** (chemokine) that specifically recruits and activates neutrophils to the site of inflammation. **Analysis of Incorrect Options:** * **IL-2 (Options A, B, D):** Primarily a T-cell growth factor. While it promotes immune responses, it is classified as a lymphocyte-stimulating cytokine rather than a primary mediator of acute inflammation. * **IL-4 (Options A, B, D):** An **anti-inflammatory** cytokine produced by Th2 cells. It inhibits the production of IL-1, IL-6, and TNF-α and promotes the "alternative activation" of macrophages (M2), which aids in tissue repair rather than inflammation. **NEET-PG High-Yield Pearls:** * **Hot T-Bone Steak** (Mnemonic for Interleukins): * **IL-1:** **Hot** (Fever) * **IL-2:** Stimulates **T**-cells * **IL-3:** **B**one marrow stimulation * **IL-4:** Ig**E** production (and IgG) * **IL-5:** Ig**A** production (and Eosinophils) * **IL-8:** "Clean up on aisle 8"—Neutrophils are recruited by IL-8 to clean up the mess. * **Anti-inflammatory cytokines:** IL-10 and TGF-β are the most potent inhibitors of the inflammatory response.

Question 782: Natural killer cells attack which of the following cells?

A. Cells which express MHC class I
B. Cells which are not able to express MHC class I (Correct Answer)
C. MHC cells which express MHC class II
D. Cells which are not able to express MHC

Explanation: **Explanation:** Natural Killer (NK) cells are large granular lymphocytes that play a critical role in the innate immune response. Their function is governed by the **"Missing Self" hypothesis**. **Why Option B is correct:** NK cells possess two types of surface receptors: **Inhibitory receptors** (e.g., KIR - Killer Immunoglobulin-like Receptors) and **Activating receptors**. * Under normal conditions, inhibitory receptors bind to **MHC Class I molecules** expressed on all healthy nucleated cells. This binding sends a "don't kill" signal, preventing autolysis. * In certain viral infections or malignancies, cells downregulate or lose MHC Class I expression to evade T-cell detection. When an NK cell encounters such a cell, the inhibitory signal is absent (the "missing self"), allowing the activating signal to trigger the release of **perforins and granzymes**, leading to apoptosis of the target cell. **Analysis of Incorrect Options:** * **Option A:** Cells expressing MHC Class I inhibit NK cell activity. These are recognized as "self" and are spared. * **Option C:** MHC Class II is primarily expressed on Professional Antigen Presenting Cells (APCs) like dendritic cells and macrophages for CD4+ T-cell activation, not for NK cell regulation. * **Option D:** This is technically imprecise. While NK cells attack cells lacking MHC, the specific trigger is the absence of **MHC Class I**, which is the ligand for KIR. **High-Yield Clinical Pearls for NEET-PG:** * **Markers:** NK cells are identified by the presence of **CD56** and **CD16** (FcγRIII) and the absence of CD3. * **Antibody-Dependent Cellular Cytotoxicity (ADCC):** CD16 allows NK cells to bind to the Fc portion of IgG, enabling them to kill antibody-coated target cells. * **Cytokine Production:** NK cells are a major source of **IFN-gamma**, which activates macrophages.

Question 783: In a patient with a Salmonella infection, which of the following mechanisms will most likely be the earliest adaptive response for clearing the infection while bacteria are present within intracellular endosomes?

A. Antibody mediated neutralization of free bacteria
B. Complement mediated lysis of infected host cells
C. Cytotoxic T-lymphocyte (CTL) recognition of bacterial peptides presented by MHC class II
D. Delayed-type hypersensitivity (DTH) responses generated by CD4+ T cells (Correct Answer)

Explanation: **Explanation:** The core concept here is the immune response to **facultative intracellular pathogens** like *Salmonella typhi*. Because these bacteria reside and replicate within the phagosomes of macrophages, they are protected from extracellular defenses. **1. Why Option D is Correct:** When *Salmonella* is sequestered within **intracellular endosomes**, the primary adaptive defense is the **Type 1 Helper T cell (Th1) response**. CD4+ T cells recognize bacterial peptides presented on **MHC Class II** molecules by infected macrophages. These T cells secrete **Interferon-gamma (IFN-γ)**, which activates the macrophages, enhancing their phagolysosomal fusion and production of reactive oxygen species (ROS) to kill the internal bacteria. This mechanism is the basis of **Delayed-Type Hypersensitivity (DTH)**. **2. Why the Other Options are Incorrect:** * **Option A:** Antibodies are effective against extracellular bacteria (neutralization/opsonization) but cannot reach or clear bacteria already residing inside host endosomes. * **Option B:** Complement-mediated lysis (MAC formation) targets the bacterial cell wall directly in the blood/extracellular fluid; it does not lyse host cells to clear intracellular infections. * **Option C:** This is a distractor. While CTLs (CD8+) do recognize infected cells, they recognize peptides presented on **MHC Class I**, not Class II. Furthermore, CTLs are more critical for viruses or bacteria that escape into the *cytosol* (e.g., *Listeria*), whereas *Salmonella* stays in the endosome. **High-Yield Clinical Pearls for NEET-PG:** * **Intracellular Pathogens:** Remember the mnemonic **"Some Bacteria Love My Lungs"** (*Salmonella, Brucella, Legionella, Mycobacterium, Listeria*)—all primarily trigger Th1/DTH responses. * **IFN-γ:** The most potent activator of macrophages; deficiency in the IFN-γ receptor leads to severe disseminated mycobacterial and salmonella infections. * **MHC Rule:** CD4+ = MHC II (Exogenous/Endosomal pathway); CD8+ = MHC I (Endogenous/Cytosolic pathway).

Question 784: Which of the following immunoglobulins mediate opsonization?

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

Explanation: **Explanation:** The correct answer is **IgE**. While IgG is the primary opsonin in systemic circulation, the question specifically targets the mechanism of **eosinophil-mediated opsonization**. 1. **Why IgE is correct:** IgE plays a specialized role in opsonizing large parasites (helminths). When IgE binds to the surface of a parasite, its Fc portion is recognized by high-affinity receptors (**FcεRI**) on eosinophils. This triggers **Antibody-Dependent Cellular Cytotoxicity (ADCC)**, leading to the release of Major Basic Protein and eosinophil cationic protein to destroy the pathogen. 2. **Why the others are incorrect:** * **IgG:** While IgG1 and IgG3 are potent opsonins for neutrophils and macrophages (via FcγR), in the context of specific exam patterns where IgE is highlighted, it refers to the unique opsonization of helminths. * **IgM:** IgM is an excellent activator of the classical complement pathway but does not act as a direct opsonin because there are no specific Fc receptors for IgM on phagocytes. * **IgA:** Primarily functions in mucosal immunity through neutralization and preventing pathogen attachment; it is not a primary opsonin. **NEET-PG High-Yield Pearls:** * **Primary Opsonins:** The two most important opsonins in the body are **IgG** and **C3b** (Complement). * **IgE Characteristics:** It is the least abundant Ig in serum, heat-labile (inactivated at 56°C for 30 mins), and mediates Type I Hypersensitivity. * **Prausnitz-Kustner (PK) Reaction:** A classic test used to demonstrate the presence of IgE (reaginic antibodies). * **Eosinophilia:** Always suspect helminthic infections or allergic conditions when IgE and eosinophil counts are elevated.

Question 785: Which statement regarding cytokines is false?

A. Mediators of inflammation
B. Does not mediate specific reactions (Correct Answer)
C. Produced by macrophages
D. Are soluble proteins

Explanation: ### Explanation Cytokines are low-molecular-weight, soluble proteins produced by various cells (primarily macrophages and T-cells) that act as chemical messengers to regulate immune responses and inflammation. **Why Option B is the Correct Answer (The False Statement):** Cytokines **do mediate specific reactions**, although they are not antigen-specific like antibodies. They exert their effects by binding to specific high-affinity receptors on target cells. This binding triggers specific intracellular signaling pathways (like the JAK-STAT pathway), leading to precise biological outcomes such as cell proliferation, differentiation, or activation. Therefore, stating they do not mediate specific reactions is incorrect. **Analysis of Other Options:** * **Option A (Mediators of inflammation):** This is true. Pro-inflammatory cytokines like **IL-1, IL-6, and TNF-α** are the primary drivers of the systemic inflammatory response and fever. * **Option C (Produced by macrophages):** This is true. Macrophages are a major source of cytokines (monokines), including TNF-α and IL-12, which bridge innate and adaptive immunity. * **Option D (Are soluble proteins):** This is true. Cytokines are secreted, soluble proteins/glycoproteins that function in an autocrine, paracrine, or endocrine fashion. **High-Yield Clinical Pearls for NEET-PG:** * **Pleiotropy:** One cytokine having multiple different effects on different cell types (e.g., IL-4 acting on B-cells, T-cells, and mast cells). * **Redundancy:** Multiple cytokines carrying out the same function (e.g., IL-2, IL-4, and IL-5 all stimulating B-cell proliferation). * **Cytokine Storm:** An overproduction of cytokines (TNF-α, IL-6) seen in conditions like COVID-19 and Septic Shock. * **IL-8:** The primary chemotactic cytokine for neutrophils ("Clean up on aisle 8").

Question 786: Regarding IgE, which of the following statements is false?

A. Causes anaphylaxis
B. Mediates immediate hypersensitivity reactions
C. Fixes complement
D. Crosses the placenta (Correct Answer)

Explanation: ### Explanation The correct answer is **D. Crosses the placenta**. This statement is false because **IgG** is the only immunoglobulin class capable of crossing the placental barrier to provide passive immunity to the fetus. #### Why the correct answer is right: IgE is a monomeric antibody with a high molecular weight and a specific Fc region that binds to mast cells and basophils rather than placental receptors (neonatal Fc receptors or FcRn). Only IgG subclasses (IgG1, IgG3, and IgG4) possess the necessary transport mechanisms to cross the placenta. #### Why the other options are wrong: * **A & B (Causes anaphylaxis / Mediates immediate hypersensitivity):** These are true statements. IgE is the primary mediator of **Type I Hypersensitivity** reactions. Upon re-exposure to an allergen, the allergen cross-links IgE molecules already bound to the surface of mast cells and basophils, leading to degranulation and the release of inflammatory mediators like histamine. * **C (Fixes complement):** This is generally considered a false property of IgE, but in the context of this multiple-choice question, "Crosses the placenta" is the *most* definitive false statement. IgE does **not** activate the classical complement pathway (only IgM and IgG do). #### High-Yield Clinical Pearls for NEET-PG: * **Heat Lability:** IgE is the most heat-labile immunoglobulin (inactivated at 56°C for 1 hour). * **Prausnitz-Küstner (PK) Reaction:** Historically used to detect IgE-mediated hypersensitivity. * **Serum Levels:** IgE has the lowest serum concentration and the shortest half-life (~2 days) among all immunoglobulins. * **Role in Parasites:** IgE plays a crucial role in immunity against helminthic infections via **Eosinophil-mediated ADCC** (Antibody-Dependent Cellular Cytotoxicity). * **Receptor:** It binds to high-affinity **FcεRI** receptors on mast cells.

Question 787: Which of the following is NOT a Type-III hypersensitivity reaction?

A. Polyarteritis nodosa
B. Post-streptococcal glomerulonephritis
C. Epstein-Barr virus infections
D. Pemphigus vulgaris (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Pemphigus vulgaris**. **1. Why Pemphigus vulgaris is the correct answer:** Pemphigus vulgaris is a **Type-II hypersensitivity reaction**. It is mediated by IgG autoantibodies directed against **Desmoglein-1 and Desmoglein-3** (cell surface antigens) in the epidermis. This leads to acantholysis (loss of cell-to-cell adhesion), resulting in intraepidermal blisters. In contrast, Type-III hypersensitivity involves the deposition of **circulating antigen-antibody (immune) complexes** in tissues, leading to complement activation and inflammation. **2. Analysis of Incorrect Options (Type-III Reactions):** * **Polyarteritis nodosa (PAN):** A classic Type-III reaction where immune complexes (often involving Hepatitis B surface antigen) deposit in small to medium-sized arteries, causing necrotizing vasculitis. * **Post-streptococcal glomerulonephritis (PSGN):** Occurs when streptococcal antigen-antibody complexes deposit in the glomerular basement membrane, leading to a "lumpy-bumpy" appearance on immunofluorescence. * **Epstein-Barr virus (EBV) infections:** Certain manifestations of EBV, such as associated vasculitis or immune-complex mediated nephritis, are classified under Type-III hypersensitivity. **3. NEET-PG Clinical Pearls:** * **Type-II Mnemonic:** "Cytotoxic" (Antibody vs. Cell Surface). Examples: Myasthenia Gravis, Goodpasture syndrome, Rheumatic fever. * **Type-III Mnemonic:** "Immune Complex" (Soluble antigen + Antibody). Examples: SLE, Arthus reaction, Serum sickness, Farmer’s Lung. * **High-Yield Fact:** Pemphigus vulgaris shows a **"row of tombstones"** appearance on histology and a **"fishnet"** pattern on direct immunofluorescence (DIF).

Question 788: Antibody diversity is due to which of the following mechanisms?

A. Gene rearrangement (Correct Answer)
B. Gene translocation
C. Antigenic variation
D. CD40 molecules

Explanation: ### Explanation **Correct Option: A. Gene rearrangement** Antibody diversity is primarily generated through **Somatic Recombination** (V(D)J recombination). Unlike most proteins encoded by a single gene, the variable regions of immunoglobulin heavy and light chains are encoded by multiple gene segments (Variable, Diversity, and Joining). During B-cell development in the bone marrow, these segments undergo random **gene rearrangement** mediated by **RAG-1 and RAG-2 enzymes**. This process, combined with junctional diversity (addition/deletion of nucleotides) and combinatorial association of heavy and light chains, allows the body to generate over $10^{11}$ unique antibody specificities from a limited number of genes. **Why other options are incorrect:** * **B. Gene translocation:** While chromosomal translocations occur in B-cells (e.g., $t(8;14)$ in Burkitt Lymphoma), they are pathological mutations rather than a physiological mechanism for normal antibody diversity. * **C. Antigenic variation:** This is a mechanism used by *pathogens* (e.g., *Neisseria gonorrhoeae*, *Trypanosoma*) to change their surface proteins to evade the host immune system; it is not a process for generating host antibodies. * **D. CD40 molecules:** CD40 (on B-cells) interacts with CD40L (on T-cells). This interaction is essential for **Isotype Switching** (Class Switch Recombination) and memory cell formation, but it does not create the initial diversity of the antigen-binding site. **High-Yield Clinical Pearls for NEET-PG:** 1. **RAG-1/RAG-2 Deficiency:** Leads to **Omenn Syndrome** or SCID (failure of V(D)J recombination). 2. **Somatic Hypermutation:** Occurs in germinal centers *after* antigen exposure, further increasing antibody affinity (Affinity Maturation). 3. **Order of Rearrangement:** Heavy chain rearranges first, followed by the Light chain. 4. **T-cell Receptor (TCR):** Also uses V(D)J rearrangement to generate diversity, similar to B-cells.

Question 789: What type of test is the Coombs test?

A. Precipitation test
B. Agglutination test (Correct Answer)
C. Complement fixation test
D. Neutralization test

Explanation: The **Coombs test (Antiglobulin test)** is a classic example of an **agglutination test**. Specifically, it is a specialized form of hemagglutination used to detect incomplete (non-agglutinating) antibodies, typically of the IgG class, against Red Blood Cells (RBCs). ### Why Agglutination is Correct: Agglutination occurs when an antibody (agglutinin) reacts with a particulate antigen (agglutinogen) on a cell surface, resulting in visible clumping. In the Coombs test, the **Coombs reagent (Antihuman Globulin/AHG)** acts as a bridge between IgG antibodies already bound to RBCs. This bridging overcomes the natural repulsive forces (zeta potential) between RBCs, leading to visible clumping (agglutination). ### Why Other Options are Incorrect: * **Precipitation test:** These involve **soluble** antigens reacting with antibodies to form an insoluble lattice. Since RBCs are large, insoluble particles, the reaction is classified as agglutination, not precipitation. * **Complement fixation test:** This measures the consumption of complement during an antigen-antibody reaction. While the Coombs test involves antibodies, it does not rely on the complement cascade for a visual result. * **Neutralization test:** These are used to determine the ability of an antibody to neutralize the biological activity of an antigen (e.g., a virus or toxin), which is not the mechanism of the Coombs test. ### High-Yield Clinical Pearls for NEET-PG: * **Direct Coombs Test (DCT):** Detects antibodies already bound to the surface of RBCs *in vivo*. Used for diagnosing Hemolytic Disease of the Newborn (HDN), Autoimmune Hemolytic Anemia (AIHA), and drug-induced hemolysis. * **Indirect Coombs Test (ICT):** Detects free antibodies in the patient’s serum *in vitro*. Used for prenatal screening (Rh incompatibility) and cross-matching before blood transfusion. * **Incomplete Antibodies:** IgG antibodies are "incomplete" because they are too small to bridge the gap between RBCs on their own; the Coombs reagent is essential to complete the lattice.

Question 790: What does a positive tuberculin test indicate?

A. Immunity to tuberculosis
B. Infection with tuberculosis
C. Cell-mediated hypersensitivity to tuberculin (Correct Answer)
D. None of the above

Explanation: ### Explanation **Correct Answer: C. Cell-mediated hypersensitivity to tuberculin** The Tuberculin Skin Test (Mantoux test) is the classic clinical example of a **Type IV (Delayed-type) Hypersensitivity reaction**. When Purified Protein Derivative (PPD) is injected intradermally, it triggers a response in individuals previously sensitized to *Mycobacterium tuberculosis*. This reaction is mediated by **T-lymphocytes** (specifically Th1 cells) rather than antibodies. These cells release cytokines that recruit macrophages, leading to local induration and erythema that peaks at **48–72 hours**. Therefore, a positive test specifically indicates that the body’s cell-mediated immune system recognizes the tuberculin antigen. **Why other options are incorrect:** * **Option A:** A positive test does **not** equate to immunity. It does not guarantee protection against future infection or reactivation; in fact, individuals with a positive test are the ones at risk for developing secondary (reactivation) TB. * **Option B:** While a positive test often suggests past or present infection, it is not definitive for "active disease." It cannot distinguish between a latent infection, a current active infection, or a prior BCG vaccination. **NEET-PG High-Yield Pearls:** * **Induration vs. Erythema:** Always measure the diameter of the **induration** (palpable raised hardening), not the erythema (redness). * **False Positives:** Most commonly caused by **BCG vaccination** or infection with Non-Tuberculous Mycobacteria (NTM). * **False Negatives (Anergy):** Can occur in malnutrition, Hodgkin’s lymphoma, sarcoidosis, miliary TB, or immunosuppression (e.g., HIV/AIDS). * **Alternative:** The **IGRA (Interferon-Gamma Release Assay)** is more specific than the Mantoux test as it is not affected by prior BCG vaccination.

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