Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 181: Which type of hypersensitivity reaction is mediated by T-lymphocytes and macrophages?

A. Type I
B. Type II
C. Type III
D. Type IV (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Type IV Hypersensitivity** **Why it is correct:** Type IV hypersensitivity is also known as **Delayed-Type Hypersensitivity (DTH)**. Unlike Types I, II, and III, which are mediated by antibodies (B-cell response), Type IV is **cell-mediated**. It involves the activation of **T-lymphocytes** (specifically Th1 and CD8+ cells). Upon re-exposure to an antigen, sensitized T-cells release cytokines (like IFN-γ) that recruit and activate **macrophages**, leading to tissue inflammation and damage. The reaction typically peaks 48–72 hours after exposure. **Why the other options are incorrect:** * **Type I (Immediate):** Mediated by **IgE antibodies** and mast cell degranulation (e.g., Anaphylaxis, Asthma). * **Type II (Cytotoxic):** Mediated by **IgG or IgM antibodies** directed against antigens on specific cell surfaces or tissues (e.g., Rh incompatibility, Myasthenia Gravis). * **Type III (Immune-complex):** Mediated by the deposition of **antigen-antibody complexes** in tissues, leading to complement activation (e.g., SLE, Post-streptococcal glomerulonephritis). **NEET-PG High-Yield Pearls:** * **Classic Examples:** Mantoux (Tuberculin) test, Contact dermatitis (poison ivy/nickel), Lepromin test, and Chronic graft rejection. * **Granuloma Formation:** This is a hallmark of persistent Type IV reactions (e.g., Tuberculosis, Sarcoidosis), where macrophages transform into **epithelioid cells** and fuse to form **multinucleated giant cells**. * **Mnemonic:** Remember **ACID** for the four types: **A**naphylactic (I), **C**ytotoxic (II), **I**mmune-complex (III), and **D**elayed/Cell-mediated (IV).

Question 182: Which of the following is an example of an agglutination test?

A. Widal test (Correct Answer)
B. Schick test
C. VDRL test
D. Ascoli's test

Explanation: **Explanation:** **1. Why Widal Test is Correct:** The **Widal test** is a classic example of a **direct slide or tube agglutination test**. It is used to diagnose Enteric fever (Typhoid) by detecting antibodies ($H$ and $O$ agglutinins) in the patient’s serum against *Salmonella typhi* and *S. paratyphi* antigens. In this reaction, particulate (insoluble) antigens clump together when they bind with specific antibodies, forming visible aggregates—the hallmark of agglutination. **2. Analysis of Incorrect Options:** * **Schick Test (Option B):** This is an **in-vivo neutralization test** used to determine susceptibility to Diphtheria. It is not an agglutination test. * **VDRL Test (Option C):** The Venereal Disease Research Laboratory (VDRL) test for Syphilis is a **flocculation test**. While similar to agglutination, flocculation involves soluble antigens that form visible "flakes" or "floccules" rather than clumps of large particles. * **Ascoli’s Test (Option D):** This is a **precipitation test** (specifically a ring precipitation test) used to detect *Bacillus anthracis* antigens in animal tissues. Precipitation involves the interaction of soluble antigens with antibodies to form an insoluble precipitate. **3. High-Yield Clinical Pearls for NEET-PG:** * **Agglutination vs. Precipitation:** Agglutination involves **particulate** antigens (e.g., bacteria, RBCs), whereas precipitation involves **soluble** antigens. * **Prozone Phenomenon:** False negatives in agglutination tests (like Widal or Brucellosis tests) can occur due to antibody excess; this is called the Prozone phenomenon. * **Coombs Test:** Another high-yield agglutination test (specifically Hemagglutination) used in Rh incompatibility and autoimmune hemolytic anemia. * **Latex Agglutination:** Commonly used for detecting CRP, RA factor, and ASO titers.

Question 183: Antisnake venom may cause which type of hypersensitivity reaction?

A. Type II hypersensitivity reactions
B. Type III hypersensitivity reactions (Correct Answer)
C. Type IV hypersensitivity reactions
D. Type V hypersensitivity reactions

Explanation: **Explanation:** **Antisnake venom (ASV)** is a classic example of **Type III hypersensitivity**, specifically manifesting as **Serum Sickness**. 1. **Why Type III is correct:** ASV is a form of passive immunization containing foreign (usually equine/horse) proteins. When injected, the body recognizes these proteins as antigens and produces antibodies (IgG/IgM). These antibodies bind to the circulating foreign proteins, forming **soluble immune complexes**. These complexes deposit in small blood vessel walls, basement membranes of joints, and kidneys, activating the complement system and causing systemic inflammation (fever, rash, arthralgia, and proteinuria). 2. **Why other options are incorrect:** * **Type II (Cytotoxic):** Involves antibodies (IgG/IgM) binding directly to antigens on **cell surfaces** or tissues (e.g., Rh incompatibility), not soluble circulating complexes. * **Type IV (Delayed-type):** This is **cell-mediated** (T-cells), occurring 48–72 hours after exposure (e.g., Mantoux test). ASV reactions are antibody-mediated. * **Type V:** Often considered a subtype of Type II, involving stimulatory antibodies (e.g., Graves' disease). **High-Yield Clinical Pearls for NEET-PG:** * **Serum Sickness** is the systemic form of Type III hypersensitivity; **Arthus Reaction** is the localized form. * **Other causes of Type III:** SLE, Post-streptococcal glomerulonephritis (PSGN), and Farmer’s lung. * **Timeline:** Serum sickness typically occurs 7–12 days after administration of the foreign serum. * **Complement levels:** Characteristically, **C3 and C4 levels are decreased** during the reaction due to massive consumption.

Question 184: Maximum lattice formation occurs in which zone?

A. Zone of antibody excess
B. Zone of antigen excess
C. Zone of equivalence (Correct Answer)
D. Can occur in any zone

Explanation: **Explanation:** The formation of an insoluble precipitate in an antigen-antibody reaction is governed by the **Marrack’s Lattice Hypothesis**. This principle states that for a visible precipitate to form, multivalent antigens and antibodies must cross-link to create a large, insoluble complex known as a **lattice**. **1. Why Zone of Equivalence is Correct:** The **Zone of Equivalence** is the point where the concentration of antigen and antibody is optimal (roughly equal). In this zone, every antigen molecule is linked to multiple antibody molecules and vice versa, leading to the formation of a large, stable, and visible lattice. This results in maximum precipitation. **2. Why Other Options are Incorrect:** * **Zone of Antibody Excess (Prozone):** When antibodies are present in very high concentrations, each antigen molecule is rapidly saturated by antibodies. This prevents the cross-linking required to form a lattice, resulting in small, soluble complexes and a false-negative result. * **Zone of Antigen Excess (Postzone):** When antigen concentration is too high, each antibody binding site is quickly occupied by a single antigen. There are not enough antibodies to bridge the antigens together, preventing lattice formation. * **Can occur in any zone:** This is incorrect because lattice formation is strictly dependent on the ratio of reactants. **NEET-PG High-Yield Pearls:** * **Prozone Phenomenon:** Clinically significant in **Syphilis (VDRL/RPR) testing** and **Brucellosis**. If a clinician suspects a disease but the test is negative, the serum should be diluted to reduce antibody concentration and reach the zone of equivalence. * **Precipitation vs. Agglutination:** Precipitation involves **soluble** antigens, while agglutination involves **particulate/insoluble** antigens (e.g., RBCs or bacteria). * **Immunodiffusion:** Techniques like Radial Immunodiffusion (Mancini) and Double Diffusion (Ouchterlony) rely on the zone of equivalence to form visible precipitation lines.

Question 185: Cytokines are released by:

A. Monocyte
B. Macrophage
C. T lymphocyte (Correct Answer)
D. B lymphocyte

Explanation: ### Explanation **Correct Answer: C. T lymphocyte** **Concept Overview:** Cytokines are low-molecular-weight regulatory proteins or glycoproteins secreted by white blood cells and various other cells in the body in response to a number of stimuli. While many cells can produce cytokines, **T lymphocytes (specifically T-helper cells)** are the primary orchestrators of the immune response and the most prolific producers of cytokines (interleukins, interferons, and TNFs) to coordinate cell-mediated and humoral immunity. **Why T lymphocyte is the best answer:** In the context of standard immunology questions, T-helper cells (CD4+) are considered the "master switches." Upon activation, they differentiate into subsets (Th1, Th2, Th17) that secrete specific profiles of cytokines (e.g., IL-2, IFN-γ, IL-4, IL-5) to signal other immune cells. **Analysis of Incorrect Options:** * **A & B (Monocytes/Macrophages):** These cells do produce cytokines (known as **monokines**, such as IL-1, IL-6, and TNF-α), but in the hierarchy of immune regulation, T-cells are the predominant source for the diverse range of cytokines that drive systemic immune responses. * **D (B lymphocyte):** B-cells primarily function as antibody-producing cells (plasma cells). While they can secrete some cytokines to modulate their own growth, it is not their primary functional characteristic compared to T-cells. **High-Yield Clinical Pearls for NEET-PG:** * **Th1 Cells:** Produce **IFN-γ and IL-2** (Cell-mediated immunity; activates macrophages). * **Th2 Cells:** Produce **IL-4, IL-5, and IL-13** (Humoral immunity; stimulates B-cells and eosinophils). * **Pleiotropy:** The ability of a single cytokine to act 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).

Question 186: Which of the following diagnostic tests are useful for corresponding purposes, except?

A. Ziehl-Neelsen staining - Detection of mycobacteria
B. Immunofluorescence - Detection of Influenza virus
C. Specific IgM antibodies - Immunity against Rubella (Correct Answer)
D. Specific IgM antibodies - Detection of acute infection

Explanation: ### Explanation The correct answer is **C (Specific IgM antibodies - Immunity against Rubella)** because it represents a mismatch between the diagnostic marker and its clinical interpretation. **1. Why Option C is the correct (incorrectly matched) choice:** In immunology, **IgM antibodies** are the first to appear following exposure to an antigen, indicating an **acute or recent infection**. They are transient and do not signify long-term protection. Conversely, **IgG antibodies** are the markers used to determine **immunity** (either from past infection or vaccination). Therefore, detecting Rubella-specific IgM indicates a current infection (often risky in pregnancy), whereas Rubella-specific IgG indicates immunity. **2. Analysis of other options:** * **Option A:** Ziehl-Neelsen (ZN) staining is the gold standard rapid bedside test for detecting Acid-Fast Bacilli (AFB) like *Mycobacterium tuberculosis*. * **Option B:** Direct Immunofluorescence (DIF) is a standard technique used to detect viral antigens (like Influenza) in respiratory secretions (nasopharyngeal swabs). * **Option C:** As established, IgM is the hallmark of the primary immune response and is the diagnostic marker for acute phase infections across most pathogens. **Clinical Pearls for NEET-PG:** * **IgM:** Does not cross the placenta (large pentamer). Its presence in a newborn indicates **congenital infection** (e.g., TORCH panel). * **IgG:** The only antibody that crosses the placenta, providing passive immunity to the fetus. * **Window Period:** The time between infection and the appearance of detectable antibodies (IgM). * **Avidity Testing:** High IgG avidity indicates an old infection, while low avidity suggests a recent infection—crucial for managing Rubella in pregnancy.

Question 187: HLA-I is present on which type of cells?

A. All nucleated cells (Correct Answer)
B. Only on cells of the immune system
C. Only on B cells
D. Only on T cells

Explanation: **Explanation:** The Human Leukocyte Antigen (HLA) system is the human version of the Major Histocompatibility Complex (MHC). These molecules are essential for the immune system to distinguish "self" from "non-self." **Why Option A is Correct:** **HLA Class I (MHC-I)** molecules are expressed on the surface of **all nucleated cells** and platelets. Their primary function is to present endogenous antigens (like viral proteins or tumor markers) to **CD8+ Cytotoxic T cells**. Since any nucleated cell in the body can potentially be infected by a virus or undergo malignant transformation, they must all possess the machinery to signal the immune system for destruction. **Why Other Options are Incorrect:** * **Options B, C, and D:** These are too restrictive. While immune cells (B cells, T cells, Macrophages) do express HLA-I, they are not the *only* cells to do so. HLA-I is ubiquitous across almost all tissues. Note that **HLA Class II**, however, has a restricted distribution and is found only on **Professional Antigen Presenting Cells (APCs)** like B cells, dendritic cells, and macrophages. **High-Yield Clinical Pearls for NEET-PG:** * **The "Rule of 8":** MHC I × CD8 = 8; MHC II × CD4 = 8. * **Structure:** HLA-I consists of a heavy chain (encoded on Chromosome 6) and a **β2-microglobulin** light chain (encoded on Chromosome 15). * **Exceptions:** Mature **Red Blood Cells (RBCs)** lack HLA-I because they are non-nucleated. This is why blood transfusions do not require HLA matching, unlike organ transplants. * **Trophoblasts:** These cells lack classical HLA-I (A and B) to prevent maternal immune rejection of the fetus.

Question 188: Mast cells release which of the following interleukins?

A. Interleukin-1
B. Interleukin-2
C. Interleukin-4
D. All of the above (Correct Answer)

Explanation: **Explanation:** Mast cells are versatile immune effector cells primarily known for their role in Type I Hypersensitivity reactions. While they are famous for releasing pre-formed mediators like histamine, they also synthesize and secrete a wide array of cytokines that modulate both innate and adaptive immunity. **Why "All of the above" is correct:** Mast cells are "multipotential" cytokine producers. * **IL-1:** Mast cells produce pro-inflammatory cytokines like IL-1 and TNF-α, which help in the recruitment of neutrophils and the initiation of the acute inflammatory response. * **IL-2:** Although primarily associated with T-cells, mast cells can produce IL-2, which aids in T-cell proliferation and regulatory T-cell (Treg) maintenance. * **IL-4:** This is a crucial cytokine produced by mast cells to drive the differentiation of Th2 cells and promote B-cell class switching to **IgE**, creating a positive feedback loop in allergic responses. **Analysis of Options:** * **IL-1:** Acts as an endogenous pyrogen and activates vascular endothelium. * **IL-2:** Known as the T-cell growth factor. * **IL-4:** Essential for the "Atopic" profile. Since mast cells have the biosynthetic machinery to produce all three, "All of the above" is the most accurate choice. **High-Yield Clinical Pearls for NEET-PG:** * **Mast Cell Markers:** CD117 (c-kit) and CD34 are key markers. * **Staining:** They show **metachromasia** with Toluidine blue (granules turn purple). * **Major Mediator:** Histamine is the primary pre-formed mediator; **Tryptase** is the most specific marker for mast cell degranulation (used clinically to diagnose anaphylaxis). * **Origin:** They originate from Hematopoietic Stem Cells (HSC) in the bone marrow but mature only after reaching peripheral tissues.

Question 189: Which of the following statements most accurately describes IgA?

A. Complement fixation tests for IgA antibody will be positive if specific IgA antibody is present.
B. IgA can be destroyed by bacterial proteases. (Correct Answer)
C. IgA is absent in colostrum.
D. IgA is not found in saliva; therefore, an IgA diagnostic test on saliva would have no value.

Explanation: **Explanation** **Correct Option: B. IgA can be destroyed by bacterial proteases.** IgA is the primary immunoglobulin responsible for mucosal immunity. To counter this defense, several pathogenic bacteria (notably *Streptococcus pneumoniae*, *Haemophilus influenzae*, and *Neisseria* species) produce a specific enzyme called **IgA1 protease**. This enzyme cleaves the proline-rich hinge region of the IgA1 molecule, effectively neutralizing its ability to prevent bacterial attachment to mucosal surfaces. **Analysis of Incorrect Options:** * **Option A:** IgA **cannot** activate the classical complement pathway. It lacks the binding site for C1q. While it can activate the alternative pathway, standard complement fixation tests (which rely on the classical pathway) will be negative for IgA. * **Option C:** IgA is the **predominant** immunoglobulin in colostrum and breast milk. It provides essential passive local immunity to the neonate's gastrointestinal tract (protection against enteric pathogens). * **Option D:** IgA is the major antibody found in **all seromucous secretions**, including saliva, tears, nasal fluids, and sweat. Salivary IgA levels can indeed be used as a diagnostic marker for certain local infections or immune deficiencies. **High-Yield Clinical Pearls for NEET-PG:** * **Structure:** Secretory IgA (sIgA) exists as a **dimer** connected by a **J-chain** and contains a **Secretory Component** (which protects it from digestive enzymes). * **Selective IgA Deficiency:** The most common primary immunodeficiency; patients are often asymptomatic but may present with recurrent sinopulmonary or GI infections and are at risk for **anaphylaxis during blood transfusions** (due to anti-IgA antibodies). * **Subclasses:** IgA1 is predominant in serum; IgA2 is more prevalent in colostrum and the lower GI tract as it is more resistant to bacterial proteases.

Question 190: Which of the following features is common to both cytotoxic T cells and NK cells?

A. Synthesizes antibodies
B. Requires antibodies to be present for action
C. Effective against virus-infected cells (Correct Answer)
D. Recognizes antigen in association with HLA class II markers

Explanation: **Explanation:** The core similarity between **Cytotoxic T cells (CD8+ T cells)** and **Natural Killer (NK) cells** lies in their primary function: the elimination of intracellular pathogens and tumor cells. Both cell types are specialized to identify and destroy **virus-infected cells** by inducing apoptosis through the release of perforins and granzymes. * **Why Option C is correct:** Both cells act as "killers" of the immune system. While CD8+ T cells are part of the adaptive immune system (requiring MHC-I presentation), and NK cells are part of the innate immune system (acting on cells that lack MHC-I), their ultimate target—cells hijacked by viruses—is the same. **Analysis of Incorrect Options:** * **Option A:** Antibody synthesis is the exclusive function of **B-lymphocytes** (specifically plasma cells). Neither T cells nor NK cells produce antibodies. * **Option B:** While NK cells can participate in Antibody-Dependent Cellular Cytotoxicity (ADCC) via CD16, it is not a *requirement* for their primary action. CD8+ T cells do not require antibodies; they require TCR-MHC interaction. * **Option D:** Recognition of antigens with **HLA Class II** is a feature of **Helper T cells (CD4+)**. Cytotoxic T cells (CD8+) recognize antigens associated with **HLA Class I**. NK cells are actually inhibited by the presence of HLA Class I (the "missing self" hypothesis). **High-Yield Clinical Pearls for NEET-PG:** * **MHC Restriction:** CD8+ T cells are MHC-restricted (Class I), whereas NK cells are **MHC-unrestricted**. * **NK Cell Markers:** Look for **CD16** (FcγRIII) and **CD56** in questions. * **The "Missing Self":** NK cells kill cells that have down-regulated MHC-I expression—a common tactic used by viruses and tumors to evade CD8+ T cells. This makes NK cells and CD8+ cells complementary.

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Antigens and Antibodies

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Major Histocompatibility Complex

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Complement System

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Cytokines and Chemokines

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Hypersensitivity Reactions

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Autoimmunity and Autoimmune Diseases

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Immunodeficiency Disorders

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Immunology — MCQs

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