Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 171: Leukotrienes are secreted by which of the following cells, except?

A. Macrophages
B. T4 cells
C. T8 cells
D. Platelets (Correct Answer)

Explanation: **Explanation:** The correct answer is **Platelets**. **1. Understanding the Core Concept:** Leukotrienes (LTs) are potent inflammatory mediators derived from **Arachidonic Acid** via the **5-Lipoxygenase (5-LOX) pathway**. The enzyme 5-LOX is primarily expressed in cells of myeloid origin, specifically **leukocytes** (neutrophils, eosinophils, basophils, monocytes/macrophages) and certain **lymphocytes**. **Platelets**, however, lack the 5-LOX enzyme. Instead, they utilize the **Cyclooxygenase (COX)** pathway to produce Thromboxane A2 and the **12-Lipoxygenase** pathway to produce 12-HETE. While platelets can participate in "transcellular biosynthesis" (taking LTA4 from neutrophils to make LTC4), they cannot synthesize leukotrienes independently from arachidonic acid. **2. Analysis of Options:** * **Macrophages (Option A):** These are myeloid cells rich in 5-LOX and are major producers of LTB4 and cysteinyl leukotrienes during chronic inflammation. * **T4 (Helper) and T8 (Cytotoxic) cells (Options B & C):** Research has confirmed that both CD4+ and CD8+ T-lymphocytes possess the enzymatic machinery to produce leukotrienes, which play a role in modulating the adaptive immune response and T-cell trafficking. **3. High-Yield Clinical Pearls for NEET-PG:** * **LTB4:** A potent **chemotactic agent** for neutrophils (mnemonic: LTB4 "B"inds neutrophils). * **LTC4, LTD4, LTE4:** Known as the **Slow-Reacting Substance of Anaphylaxis (SRS-A)**; they cause intense bronchoconstriction and increased vascular permeability. * **Zileuton:** A drug that inhibits 5-Lipoxygenase. * **Montelukast/Zafirlukast:** Leukotriene receptor antagonists (specifically for the CysLT1 receptor) used in asthma management.

Question 172: Which are the gene components of HLA class I?

A. A, B, C (Correct Answer)
B. DR
C. DQ
D. DP

Explanation: The Human Leukocyte Antigen (HLA) system is the Major Histocompatibility Complex (MHC) in humans, located on the short arm of **Chromosome 6**. It is divided into three classes based on structure and function. ### **Explanation of the Correct Answer** **Option A (A, B, C)** is correct because these genes encode the **HLA Class I** molecules. * **Structure:** Class I molecules consist of a heavy chain (encoded by HLA-A, B, or C genes) non-covalently linked to a **$\beta_2$-microglobulin** (encoded on Chromosome 15). * **Distribution:** They are expressed on almost all **nucleated cells** and platelets (but not on mature RBCs). * **Function:** They present endogenous antigens to **CD8+ Cytotoxic T-cells**. ### **Explanation of Incorrect Options** **Options B, C, and D (DR, DQ, DP)** are incorrect because these genes encode **HLA Class II** molecules. * **Distribution:** Unlike Class I, Class II molecules are restricted to **Antigen-Presenting Cells (APCs)** such as macrophages, B-cells, and dendritic cells. * **Function:** They present exogenous antigens to **CD4+ Helper T-cells**. ### **High-Yield Clinical Pearls for NEET-PG** * **MHC Restriction:** CD8+ cells recognize Class I (Rule of 8: 8 × 1 = 8), while CD4+ cells recognize Class II (4 × 2 = 8). * **HLA-B27:** Strongly associated with Seronegative Spondyloarthropathies (e.g., Ankylosing Spondylitis). * **HLA-DR3/DR4:** Associated with Type 1 Diabetes Mellitus. * **HLA-B*5701:** Screening is mandatory before starting Abacavir (HIV) to prevent hypersensitivity. * **Class III MHC:** Includes genes for Complement components (C2, C4) and TNF (Tumor Necrosis Factor).

Question 173: Which statement about IgM antibody is FALSE?

A. Has the highest molecular weight
B. Has the highest sedimentation coefficient
C. It is monomeric in nature (Correct Answer)
D. IgM is responsible for agglutination

Explanation: **Explanation:** **1. Why Option C is the Correct (False) Statement:** IgM is the largest immunoglobulin and primarily exists as a **pentamer** (five monomer units) in its secreted form, held together by a **J-chain** (Joining chain) and disulfide bonds. While IgM is found as a monomer on the surface of B-cells (acting as a B-cell receptor), its characteristic physiological form in serum is pentameric. Therefore, stating it is "monomeric in nature" as a general rule is incorrect. **2. Analysis of Incorrect Options:** * **Option A (Highest Molecular Weight):** True. Due to its pentameric structure, IgM has a molecular weight of approximately **900,000 Daltons (900 kDa)**, making it the "Millionaire Molecule." * **Option B (Highest Sedimentation Coefficient):** True. Because of its large size and mass, it has the highest sedimentation coefficient, typically **19S** (compared to 7S for IgG). * **Option C (Responsible for Agglutination):** True. With **10 theoretical antigen-binding sites** (valency of 10, though effectively 5 due to steric hindrance), IgM is highly efficient at cross-linking particulate antigens, making it the most potent antibody for agglutination and complement fixation. **3. High-Yield Clinical Pearls for NEET-PG:** * **First Responder:** IgM is the first antibody to appear in response to an initial exposure to an antigen (Primary Immune Response). * **Acute Infection Marker:** Presence of specific IgM in serum indicates a **recent/acute infection** or congenital infection (as IgM cannot cross the placenta). * **Evolutionary Fact:** It is the oldest class of immunoglobulins phylogenetically. * **Intravascular Distribution:** Due to its large size, it is largely confined to the intravascular compartment (80%).

Question 174: Which components of innate immunity are active against viral cells?

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

Explanation: **Explanation:** The core of this question lies in distinguishing between **innate** and **adaptive** immunity. **Why NK cells are correct:** Natural Killer (NK) cells are a critical component of the **innate immune system**. Unlike T or B cells, they do not require prior sensitization or MHC-restricted antigen presentation to function. They are specifically designed to target virally infected cells and tumor cells. NK cells detect "missing self" (downregulation of MHC-I molecules, a common viral evasion strategy) and release perforins and granzymes to induce apoptosis in the target cell. They also secrete Interferon-gamma (IFN-γ) to activate macrophages. **Why the other options are incorrect:** * **Cytotoxic T cells (CD8+):** While these are highly effective against viruses, they belong to the **adaptive immune system**. They require specific antigen presentation via MHC-I and take time to proliferate during a primary infection. * **B cells:** These are part of the **adaptive humoral immunity**. Their primary role is to differentiate into plasma cells that produce antibodies. * **Memory B cells:** These are specialized adaptive immune cells formed after an initial infection to provide rapid, long-term protection upon re-exposure. **High-Yield NEET-PG Pearls:** * **NK Cell Markers:** CD16 (FcγRIII, binds IgG for ADCC) and CD56 (NCAM) are the definitive markers. * **ADCC:** NK cells participate in Antibody-Dependent Cellular Cytotoxicity via the CD16 receptor. * **Cytokine Link:** IL-12 and IL-15 are potent activators of NK cells. * **MHC-I:** NK cells are *inhibited* by the presence of normal MHC-I; viruses that downregulate MHC-I to hide from T cells become primary targets for NK cells.

Question 175: Primary antibody deficiencies are characterized by which of the following?

A. Recurrent allergic reactions
B. Recurrent bacterial infections (Correct Answer)
C. Implicit allergic reactions
D. Implicit bacterial infections

Explanation: **Explanation:** Primary antibody deficiencies (B-cell defects) are the most common type of primary immunodeficiency disorders, accounting for approximately 50–60% of cases. **Why Option B is Correct:** Antibodies (Immunoglobulins) are essential for the **opsonization** of encapsulated bacteria, neutralization of toxins, and activation of the complement system. A deficiency in B-cell maturation or function (e.g., X-linked Agammaglobulinemia or CVID) leads to a failure in these mechanisms. Consequently, patients present with **recurrent bacterial infections**, typically involving the respiratory tract (sinusitis, pneumonia, otitis media). The most common pathogens involved are encapsulated organisms like *Streptococcus pneumoniae* and *Haemophilus influenzae*. **Why Other Options are Incorrect:** * **Options A & C:** Allergic reactions are mediated by an overactive immune response (specifically IgE and Th2 cells). In antibody deficiencies, the immune system is underactive/hypofunctional, making recurrent allergies an unlikely primary characteristic. * **Option D:** The term "implicit" is medically inaccurate in this context. Clinical presentations of immunodeficiencies are "explicit" or overt clinical manifestations (recurrent, severe, or persistent infections). **High-Yield Clinical Pearls for NEET-PG:** * **Timing:** Symptoms usually appear after **6 months of age**, once maternal IgG (transferred placentally) wanes. * **Commonest Type:** **Selective IgA deficiency** is the most common primary immunodeficiency; patients are often asymptomatic but may have anaphylaxis during blood transfusions. * **X-linked Agammaglobulinemia (Bruton’s):** Characterized by a defect in **BTK (Bruton Tyrosine Kinase)**, leading to a total absence of B-cells and all classes of antibodies. * **Diagnosis:** Initial screening involves measuring serum quantitative immunoglobulin levels (IgG, IgA, IgM).

Question 176: Asthma is a result of which type of hypersensitivity?

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

Explanation: **Explanation:** **Correct Answer: D (Type IV Hypersensitivity)** While Bronchial Asthma is classically associated with Type I hypersensitivity (IgE-mediated), chronic asthma—characterized by persistent airway inflammation, remodeling, and eosinophil recruitment—is fundamentally a **Type IV (Delayed-type) Hypersensitivity** reaction. This phase is mediated by **Th2 cells**, which secrete cytokines like IL-4, IL-5, and IL-13. These cytokines drive the chronic inflammatory response and late-phase reaction, making Type IV the most accurate description of the underlying pathology in chronic cases. **Analysis of Incorrect Options:** * **Type I (Immediate):** This involves IgE binding to mast cells, leading to degranulation and histamine release. While this triggers the *acute* bronchospasm (early phase), it does not account for the chronic inflammatory damage. * **Type II (Cytotoxic):** This involves IgG/IgM antibodies directed against antigens on cell surfaces (e.g., Goodpasture syndrome). It plays no role in the pathogenesis of asthma. * **Type III (Immune-complex):** This involves the deposition of antigen-antibody complexes in tissues (e.g., SLE or Arthus reaction). This mechanism is not involved in asthma. **High-Yield Clinical Pearls for NEET-PG:** * **Gell and Coombs Classification:** Remember that many diseases involve multiple types. Asthma is **Type I (Acute)** and **Type IV (Chronic)**. * **Key Cytokine:** **IL-5** is the most specific cytokine for eosinophil recruitment in asthma. * **Charcot-Leyden Crystals:** Formed from the breakdown of eosinophils, often seen in the sputum of asthmatic patients. * **Curschmann Spirals:** Whorled mucus plugs found in subepithelial mucous gland ducts in asthma.

Question 177: A hybridoma cell line is a product of fusion between which of the following?

A. B cells with mouse myeloma cells (Correct Answer)
B. T cells with mouse myeloma cells
C. B cells with Guinea pig myeloma cells
D. T cells with guinea pig myeloma cells

Explanation: ### Explanation **Concept Overview:** The **Hybridoma Technique**, developed by Köhler and Milstein, is the cornerstone for producing **Monoclonal Antibodies (mAbs)**. The goal is to create a cell line that possesses two specific properties: the ability to produce a specific antibody and the ability to multiply indefinitely (immortality). **Why Option A is Correct:** A hybridoma is formed by the fusion of: 1. **B cells (Splenic Lymphocytes):** These are sourced from a mouse immunized with a specific antigen. They provide the genetic machinery to produce the desired antibody but have a short lifespan. 2. **Mouse Myeloma Cells:** These are cancerous plasma cells. They are "immortal" (divide indefinitely) but are specifically chosen to be non-secretory (do not produce their own antibodies) and HGPRT-deficient (allowing for selection in HAT medium). The resulting fusion creates a "hybrid" cell that is an immortal antibody factory. **Analysis of Incorrect Options:** * **Options B & D:** T cells are involved in cell-mediated immunity and do not produce antibodies. Fusing T cells would not result in monoclonal antibody production. * **Options C & D:** While theoretically possible in research, the standard, commercially established, and high-yield protocol for the NEET-PG exam specifically utilizes **mouse** (murine) myeloma cells. **High-Yield Facts for NEET-PG:** * **Selection Medium:** **HAT Medium** (Hypoxanthine, Aminopterin, Thymidine) is used to select only the fused hybridoma cells. Unfused myeloma cells die because they lack the HGPRT enzyme, and unfused B cells die naturally due to their short lifespan. * **Nobel Prize:** Köhler and Milstein received the Nobel Prize in 1984 for this discovery. * **Clinical Use:** Monoclonal antibodies are used in diagnostics (ELISA, Western Blot) and therapeutics (e.g., Rituximab, Infliximab).

Question 178: When an antigen is administered for the first time to an animal or a human being who has never been exposed to it, what is the first antibody to develop?

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

Explanation: **Explanation:** In the **Primary Immune Response**, which occurs when the body encounters a specific antigen for the first time, there is a characteristic sequence of antibody production. After a latent period (lag phase) of 5–10 days, **IgM** is the first class of immunoglobulin to appear. This is because IgM is the "default" antibody produced by naive B cells before they undergo **class-switching**. IgM is a pentamer, providing high avidity to neutralize pathogens early in the infection. **Analysis of Options:** * **Option A (IgG):** IgG is the predominant antibody in the **Secondary Immune Response** (anamnestic response). While it follows IgM in the primary response after class-switching, it is not the *first* to develop. IgG is the only antibody that crosses the placenta. * **Option C (IgA):** This is the primary secretory antibody found in colostrum, saliva, and mucosal surfaces. It provides local immunity but is not the primary systemic responder to a new antigen. * **Option D (IgE):** This antibody is primarily involved in Type I hypersensitivity (allergic) reactions and defense against helminthic parasitic infections. **High-Yield NEET-PG Pearls:** * **IgM:** Largest antibody (Macroglobulin), first to appear in evolution and phylogeny, and indicates **acute/recent infection**. It does not cross the placenta; thus, its presence in a newborn indicates intrauterine infection (e.g., TORCH). * **Lag Phase:** Shorter in secondary response (1–3 days) compared to primary response (5–10 days). * **Affinity Maturation:** Occurs during the transition from IgM to IgG, leading to higher antigen-binding strength in subsequent exposures.

Question 179: In tuberculosis, which cytokine plays a major role in the conversion of tissue macrophages into epithelioid cells?

A. Interferon gamma (Correct Answer)
B. Tumor necrosis factor
C. Interleukin 12
D. Macrophage chemoattractant protein

Explanation: **Explanation:** The formation of a granuloma in Tuberculosis is a classic example of **Type IV (Delayed-type) Hypersensitivity**. The conversion of macrophages into epithelioid cells is the hallmark of this process. **Why Interferon-gamma (IFN-γ) is correct:** In response to *Mycobacterium tuberculosis* antigens, CD4+ T-cells (Th1 subset) secrete **IFN-γ**. This is the most potent activator of macrophages. Under the chronic influence of IFN-γ, macrophages undergo structural changes: they increase their cytoplasmic volume, develop abundant organelles, and take on a "slipper-shaped" appearance resembling epithelial cells—hence the term **epithelioid cells**. These cells have enhanced bactericidal activity but are primarily secretory and structural in nature. **Analysis of Incorrect Options:** * **Tumor Necrosis Factor (TNF-α):** While TNF-α is crucial for **maintaining** the structural integrity of the granuloma and preventing hematogenous spread, it is not the primary driver of epithelioid transformation. * **Interleukin-12 (IL-12):** This cytokine is secreted by macrophages to stimulate T-cells and NK cells to produce IFN-γ. It acts "upstream" in the pathway. * **Macrophage Chemoattractant Protein (MCP-1):** This chemokine is responsible for the **recruitment** of monocytes from the blood to the site of infection, not their differentiation into epithelioid cells. **High-Yield Clinical Pearls for NEET-PG:** * **Epithelioid cells** are defined by their inability to phagocytose but high secretory capacity. * Fusion of epithelioid cells under the influence of IFN-γ leads to the formation of **Langhans giant cells** (peripheral arrangement of nuclei). * **Key Cytokine Sequence:** IL-12 (from Macrophage) → IFN-γ (from Th1 cell) → Macrophage activation/Epithelioid change. * Anti-TNF drugs (like Infliximab) carry a high risk of **reactivating latent TB** because they disrupt existing granulomas.

Question 180: What is the origin of the secretory component of an immunoglobulin molecule?

A. Formed by epithelial cells of the lining mucosa
B. Formed by plasma cells
C. Formed by both epithelial cells and plasma cells (Correct Answer)
D. Secreted by bone marrow

Explanation: ### Explanation The secretory component is a unique structural feature of **Secretory IgA (sIgA)**, the primary immunoglobulin found in mucosal secretions (tears, saliva, colostrum, and intestinal mucus). **Why Option C is Correct:** The formation of Secretory IgA is a collaborative process involving two different cell types: 1. **Plasma Cells:** Located in the *lamina propria*, these cells synthesize the **IgA monomer** and the **J-chain** (Joining chain). Two IgA monomers are linked by the J-chain to form **Dimeric IgA**. 2. **Epithelial Cells:** These cells express a receptor called the **poly-Ig receptor** on their basolateral surface. The Dimeric IgA binds to this receptor and is internalized via transcytosis. During transport to the apical surface, the receptor is cleaved, leaving a fragment attached to the IgA molecule. This fragment is the **Secretory Component**. Therefore, while the IgA itself comes from plasma cells, the secretory component is derived from the epithelial receptor. **Analysis of Incorrect Options:** * **Option A:** Epithelial cells only provide the secretory component; they do not produce the IgA molecule itself. * **Option B:** Plasma cells produce the IgA and J-chain but do not produce the secretory component. * **Option D:** Bone marrow is the site of hematopoiesis and systemic IgA production, but secretory IgA is specifically processed at mucosal surfaces. **High-Yield NEET-PG Pearls:** * **Function of Secretory Component:** It protects the IgA molecule from proteolytic digestion by enzymes in the gut and respiratory tract. * **Valency:** Secretory IgA is a **dimer** and has **4 antigen-binding sites**. * **J-Chain:** Also found in **IgM** (which is a pentamer). * **Selective IgA Deficiency:** The most common primary immunodeficiency; patients often present with recurrent sinopulmonary or GI infections.

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Cells and Organs of Immune System

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Innate Immunity

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Adaptive Immunity

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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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Transplantation Immunology

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Tumor Immunology

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Immunology — MCQs

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