Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 1001: What is the Oakley-Fulthorpe procedure primarily used for?

A. Determining blood type
B. Measuring hormone levels
C. Identifying bacterial infections
D. Detection of specific antibodies in serum (Correct Answer)

Explanation: ***Detection of specific antibodies in serum*** - The **Oakley-Fulthorpe procedure** is an **immunodiffusion technique** used to detect and identify **precipitating antibodies** in serum. - It is a modification of the **Ouchterlony double diffusion method**, performed in **agar gel** where antigen and antibody solutions diffuse toward each other. - When specific antibody-antigen complexes form, they create visible **precipitin lines** in the gel, allowing for identification and comparison of antibodies. - This technique is particularly useful for detecting **antibodies in serum** against specific antigens. *Determining blood type* - Blood typing is performed using **agglutination assays** (e.g., forward and reverse typing) to identify ABO and Rh antigens on red blood cells. - This involves direct mixing of blood with antisera, not immunodiffusion techniques. *Measuring hormone levels* - Hormone levels are measured using **immunoassays** such as ELISA (Enzyme-Linked Immunosorbent Assay), radioimmunoassays (RIA), or chemiluminescence assays. - These methods quantify hormone concentrations directly, not through diffusion-based precipitin reactions. *Identifying bacterial infections* - Bacterial infections are diagnosed through **culture and sensitivity testing**, microscopy, molecular methods (PCR), or rapid antigen tests. - While immunodiffusion can detect antibodies against bacterial antigens, it is not the primary method for diagnosing active bacterial infections in clinical practice.

Question 1002: What is required for precipitation in comparison to agglutination?

A. Soluble antigen (Correct Answer)
B. Increased temperature
C. Specific cofactor
D. Lower pH

Explanation: ***Soluble antigen*** - **Precipitation reactions** involve the interaction of antibodies with **soluble antigens** to form a detectable precipitate. - Unlike **agglutination**, which involves particulate antigens (e.g., cells), precipitation requires the antigen to be dissolved in a solution. *Increased temperature* - Most immunologic reactions, including precipitation and agglutination, are typically performed at **physiological temperatures** (e.g., 37°C) or room temperature. - An **increased temperature** is not a specific requirement that differentiates precipitation from agglutination. *Specific cofactor* - While some complex immunologic reactions might require **cofactors**, neither precipitation nor agglutination inherently requires a specific cofactor to occur. - The primary components are **antigen** and **antibody**. *Lower pH* - Both precipitation and agglutination reactions are sensitive to pH and typically occur within a **narrow pH range** close to neutral (e.g., pH 7.0-7.4). - A **lower pH** (acidic environment) could lead to antibody denaturation or non-specific aggregation, ultimately hindering the reaction rather than being a requirement.

Question 1003: Which of the following is an example of a neutralization reaction?

A. VDRL
B. Widal test
C. Kahn test
D. Nagler reaction (Correct Answer)

Explanation: ***Nagler reaction*** - The Nagler reaction is a **neutralization test** used to detect α-toxin (lecithinase) produced by *Clostridium perfringens*. - A positive result occurs when Lecithinase activity is **neutralized by antitoxin**, preventing hydrolysis of lecithin in egg yolk agar. *VDRL* - The **Venereal Disease Research Laboratory (VDRL) test** is a flocculation test used for screening syphilis, detecting cardiolipin antibodies. - It involves the clumping of **antigen-antibody complexes**, not the neutralization of a toxin. *Widal test* - The **Widal test** is an agglutination test for the diagnosis of enteric fever (typhoid), detecting antibodies against *Salmonella* O and H antigens. - It identifies bacterial agglutination, which is the **clumping of bacterial cells** in the presence of specific antibodies. *Kahn test* - The **Kahn test** is another flocculation test, similar to VDRL, historically used for syphilis screening. - It involves the precipitation of a lipid antigen by syphilitic antibodies, which is an **agglutination phenomenon**, not neutralization.

Question 1004: Interleukin 2 is produced by

A. T helper cells 1 (Correct Answer)
B. T helper cells 2
C. Natural killer cells
D. Basophils

Explanation: ***T helper cells 1*** - **T helper 1 (Th1) cells** are a primary source of **interleukin-2 (IL-2)**, which is crucial for the proliferation and survival of T cells. - IL-2 acts as a **T-cell growth factor**, promoting the expansion of activated T cells, including cytotoxic T lymphocytes. *T helper cells 2* - **T helper 2 (Th2) cells** primarily produce cytokines like **IL-4, IL-5, IL-6, IL-10, and IL-13**, which are involved in humoral immunity and allergic responses. - While Th2 cells are important for immune responses, they are not major producers of IL-2. *Natural killer cells* - **Natural killer (NK) cells** are part of the innate immune system and produce cytokines such as **interferon-gamma (IFN-$\gamma$)** and **tumor necrosis factor-alpha (TNF-$\alpha$)**. - They are not a significant source of IL-2, which is primarily a T-cell derived growth factor. *Basophils* - **Basophils** are granulocytes involved in allergic reactions and anti-parasitic immunity, producing mediators like **histamine** and cytokines such as **IL-4** and **IL-13**. - Basophils do not produce IL-2; their role is distinct in the immune response compared to T cells.

Question 1005: The process by which antigen-specific B lymphocytes are selected and activated to proliferate and produce antibodies is called:

A. Clonal selection (Correct Answer)
B. Class switching
C. Group switching
D. Hybridisation

Explanation: ***Clonal selection*** - **Clonal selection** is the fundamental process by which an antigen-specific B lymphocyte is **selected** when its B cell receptor (BCR) recognizes and binds to a matching antigen. - This binding triggers the B cell to become **activated**, **proliferate** (undergo clonal expansion), and **differentiate** into plasma cells that produce antibodies specific to that antigen. - This process is the cornerstone of **adaptive immunity**, ensuring that only B cells with receptors matching the encountered antigen are stimulated to respond. *Class switching* - **Class switching** (isotype switching) occurs AFTER clonal selection and activation. - It allows already-activated B cells to change the **antibody class** they produce (from IgM to IgG, IgA, or IgE) while maintaining the **same antigen specificity**. - This process modifies effector functions but does NOT involve the initial selection and activation of antigen-specific B cells. *Group switching* - This is not a recognized term in immunology. - It does not describe any standard process of B cell activation or antibody production. *Hybridisation* - **Hybridization** refers to the formation of double-stranded nucleic acids from complementary strands or the creation of hybrid cells (e.g., hybridomas for monoclonal antibody production). - It is not the physiological process by which B lymphocytes are selected and activated in response to antigen exposure.

Question 1006: In the context of immune response, which of the following cell types does not express MHC class II molecules?

A. Cortical macrophages
B. Neutrophils
C. Medullary macrophages
D. NK cells (Correct Answer)

Explanation: ***NK cells*** - **Natural Killer (NK) cells)** are innate lymphocytes that do **NOT express MHC class II molecules** under any circumstances. - NK cells use alternative recognition mechanisms (KIRs, activating receptors) to detect target cells, primarily recognizing the **absence of MHC class I** or stress-induced ligands. - They function in innate immunity without antigen presentation capability. - **This is the best answer** as NK cells never express MHC class II, making them distinctly different from professional APCs. *Cortical macrophages* - **Cortical macrophages** in lymphoid organs are professional **antigen-presenting cells (APCs)** that constitutively express **MHC class II molecules**. - They present processed antigens to CD4+ T helper cells, playing a crucial role in initiating adaptive immune responses. *Medullary macrophages* - **Medullary macrophages** are also professional APCs that constitutively express **MHC class II molecules**. - They participate in antigen presentation and immune surveillance within the medullary regions of lymphoid tissues. *Neutrophils* - Neutrophils are granulocytes that **typically do not constitutively express MHC class II molecules** in their resting state. - However, under certain inflammatory conditions with prolonged stimulation (IFN-γ, GM-CSF), neutrophils can be induced to express low levels of MHC class II. - While neutrophils generally lack MHC class II, **NK cells are the more definitive answer** as they never express MHC class II under any physiological or pathological conditions.

Question 1007: IFN-gamma is produced by

A. Macrophages
B. T-cells (Correct Answer)
C. Neutrophils
D. B-cells

Explanation: ***T-cells*** - **Interferon-gamma (IFN-γ)** is a crucial cytokine primarily produced by **activated T-lymphocytes**, especially **Th1 cells** and **cytotoxic T lymphocytes (CTLs)**. - Natural killer (NK) cells also produce **IFN-γ**, which plays a key role in **antiviral** and **antitumor immunity**, as well as in promoting **Type 1 immune responses**. *Macrophages* - While macrophages are **responsive to IFN-γ** (e.g., becoming activated), they are not the primary producers of this cytokine. - Macrophages primarily produce other cytokines such as **IL-1, IL-6, TNF-alpha**, and **IL-12** in response to infection or inflammation. *Neutrophils* - **Neutrophils** are key phagocytes in the innate immune system and are primarily involved in engulfing and killing pathogens. - They are not known to be a significant source of **IFN-γ** production; their main defensive mechanisms involve **phagocytosis**, **degranulation**, and **NETosis**. *B-cells* - **B-cells** are central to humoral immunity, specializing in **antibody production** and acting as **antigen-presenting cells**. - They generally do not produce **IFN-γ**; instead, their cytokine repertoire includes **IL-10**, **IL-6**, and **lymphotoxin**.

Question 1008: Rosette formation with sheep RBCs (SRBCs) indicates functioning of -

A. T-cells (Correct Answer)
B. B-cells
C. Neutrophils
D. Monocytes

Explanation: ***T-cells*** - **T-cells** possess specific receptors, like **CD2** on their surface, that can bind to ligands on sheep red blood cells (SRBCs). - This binding leads to the formation of characteristic **rosettes**, where SRBCs cluster around the T-lymphocytes, indicating functional T-cells. *B-cells* - **B-cells** primarily function in **humoral immunity** by producing antibodies and do not typically form rosettes with sheep RBCs. - While B-cells have surface receptors, they are not CD2 and thus do not facilitate this specific type of rosette formation. *Neutrophils* - **Neutrophils** are **phagocytic cells** involved in innate immunity, primarily combating bacterial and fungal infections. - They lack the specific surface receptors (like CD2) required to form rosettes with sheep RBCs. *Monocytes* - **Monocytes** are precursors to macrophages and dendritic cells, involved in phagocytosis and antigen presentation. - They do not possess the necessary surface markers to form rosettes with sheep RBCs.

Question 1009: What is the most common genetic factor associated with increased susceptibility to Neisseria infections?

A. HLA-B27
B. Complement deficiency (Correct Answer)
C. IgA deficiency
D. Factor H deficiency

Explanation: ***Complement deficiency*** - Deficiencies in the **terminal complement pathway (C5-C9)**, particularly C5b-C9 (membrane attack complex, MAC), significantly increase susceptibility to disseminated *Neisseria* infections. - The MAC is crucial for lysing Gram-negative bacteria like *Neisseria meningitidis* and *Neisseria gonorrhoeae*, and its absence allows for uncontrolled bacterial proliferation. *Factor H deficiency* - **Factor H** is a regulatory protein of the alternative complement pathway, preventing its overactivation on host cells. - Its deficiency typically leads to conditions like **atypical hemolytic uremic syndrome (aHUS)** and **dense deposit disease**, not primarily increased susceptibility to *Neisseria* infections. *HLA B27* - **HLA-B27** is a human leukocyte antigen strongly associated with a group of autoimmune inflammatory diseases called **spondyloarthropathies**, such as ankylosing spondylitis. - It does not directly impact the immune response to *Neisseria* infections or increase susceptibility to them. *IgA deficiency* - **Selective IgA deficiency** is the most common primary immunodeficiency, characterized by low or absent IgA levels. - Individuals with IgA deficiency are more prone to **recurrent respiratory and gastrointestinal infections**, but not specifically disseminated *Neisseria* infections.

Question 1010: Cytolytic activity of membrane attack complex is modulated by ?

A. Factor I
B. Factor B
C. Factor S (vitronectin) (Correct Answer)
D. Factor H

Explanation: ***Correct Option: Factor S (vitronectin)*** - Vitronectin (S-protein) is a **plasma protein** that directly modulates the **cytolytic activity of the membrane attack complex (MAC)**. - It binds to the **C5b-7 complex** in the fluid phase, preventing its insertion into target cell membranes and thereby blocking the formation of the complete, functional MAC. - By inhibiting membrane insertion of C5b-7, vitronectin prevents the subsequent binding of **C8 and C9**, which are essential for the cytolytic pore formation. - This is a **direct modulation** of MAC's cytolytic activity at the MAC assembly stage. *Incorrect Option: Factor H* - Factor H is a regulatory protein that controls the **alternative pathway** of complement activation by promoting degradation of **C3b**. - By degrading C3b, Factor H prevents formation of **C5 convertase**, thereby reducing downstream MAC formation. - However, Factor H acts **early in the complement cascade** and does not directly modulate the cytolytic activity of already-formed MAC components. - Its effect is on **preventing MAC formation**, not on modulating MAC's cytolytic function itself. *Incorrect Option: Factor I* - Factor I is a **serine protease** that cleaves and inactivates C3b and C4b, requiring cofactors like Factor H or C4bp. - Like Factor H, it regulates complement activation **upstream** of MAC formation. - It does not directly interact with or modulate the cytolytic activity of the MAC. *Incorrect Option: Factor B* - Factor B is a component of the **alternative pathway C3 convertase** (C3bBb). - It **promotes complement activation** rather than modulating MAC's cytolytic activity. - Factor B functions early in the cascade and has no direct role in regulating MAC function.

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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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Complement System

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Immunology — MCQs

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