Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 431: Which is a Pan T-cell marker?

A. CD3 (Correct Answer)
B. CD8
C. CD45
D. CD30

Explanation: **Explanation:** **CD3** is the definitive **Pan T-cell marker** because it is physically associated with the T-cell receptor (TCR) complex. It is required for the cell-surface expression of the TCR and for signaling after the TCR binds to an antigen. Since CD3 is expressed on all mature T-lymphocytes (both helper and cytotoxic), it is used in flow cytometry and immunohistochemistry to identify the T-cell lineage. **Analysis of Incorrect Options:** * **CD8:** This is a marker for **Cytotoxic T-cells** and is also found on a subset of Natural Killer (NK) cells. It is not a "pan" marker because it is absent on Helper T-cells (CD4+). * **CD45:** Known as the **Leukocyte Common Antigen (LCA)**. It is expressed on all white blood cells (neutrophils, lymphocytes, monocytes, etc.), not just T-cells. * **CD30:** This is a marker of activated T and B cells. It is clinically significant as a diagnostic marker for **Reed-Sternberg cells** in Hodgkin Lymphoma and Anaplastic Large Cell Lymphoma (ALCL). **High-Yield Clinical Pearls for NEET-PG:** * **Pan B-cell markers:** CD19 (earliest), CD20, and CD21. * **NK cell markers:** CD16 and CD56. * **HSC marker:** CD34 (Hematopoietic Stem Cell). * **T-cell maturation:** CD3 appears in the cytoplasm of pro-thymocytes before it is expressed on the surface of mature T-cells. * **Therapeutic link:** Muromonab-CD3 (OKT3) is a monoclonal antibody targeting CD3 used to prevent organ transplant rejection.

Question 432: Which of the following is an autoantigen?

A. Blood group antigen
B. Forssman antigen
C. Both of the above
D. Neither of the above (Correct Answer)

Explanation: ### Explanation To answer this question, we must first define the types of antigens based on their origin relative to the host: 1. **Autoantigens:** These are "self-antigens" present on an individual’s own cells. Under normal conditions, the immune system is tolerant of them, but in autoimmune diseases, they trigger an immune response. 2. **Isoantigens (Alloantigens):** These are present in some but not all members of the same species (e.g., Blood group antigens, HLA). 3. **Heteroantigens:** These are found in different species (e.g., plant, animal, or microbial antigens). 4. **Heterophile Antigens:** A subset of heteroantigens that are identical or closely related across different species, leading to cross-reactivity. #### Analysis of Options: * **Option A (Blood group antigen):** These are **Isoantigens**. While they are "self" to the individual carrying them, they are classified as isoantigens because they vary among members of the same species (Human A vs. Human B). * **Option B (Forssman antigen):** This is a classic **Heterophile antigen**. It is a lipid-carbohydrate complex found in the tissues of many species (guinea pigs, horses, sheep) but notably absent in humans and rabbits. It is used in diagnostic tests based on cross-reactivity. * **Option D (Correct):** Since neither A nor B fits the definition of an autoantigen, "Neither of the above" is the correct choice. #### NEET-PG High-Yield Pearls: * **Heterophile Antigens:** The most famous clinical application is the **Paul-Bunnell Test** for Infectious Mononucleosis (EBV), which detects heterophile antibodies that agglutinate sheep RBCs. * **Weil-Felix Reaction:** Another example of heterophile reactivity where antibodies against *Rickettsia* cross-react with *Proteus* antigens (OX-19, OX-2, OX-K). * **Autoantigens in Disease:** Examples include native DNA in SLE or thyroglobulin in Hashimoto’s thyroiditis.

Question 433: Polysaccharide antigens are:

A. T cell independent antigens (Correct Answer)
B. T cell dependent antigens
C. MHC I dependent antigens
D. MHC II dependent antigens

Explanation: ### Explanation **1. Why Option A is Correct:** Polysaccharide antigens (such as those found in the capsules of *S. pneumoniae* or *H. influenzae*) are classified as **T-cell Independent (TI) antigens**. Unlike proteins, polysaccharides cannot be processed and presented on MHC molecules to T-cells. Instead, they contain large, repeating identical epitopes that can **cross-link multiple B-cell receptors (BCRs)** simultaneously. This cross-linking provides a signal strong enough to activate B-cells directly to produce antibodies (mainly IgM) without the "help" of T-helper cells. **2. Why the Other Options are Incorrect:** * **Option B (T-cell Dependent):** These are typically **protein antigens**. They require processing by Antigen Presenting Cells (APCs) and presentation to T-helper cells via MHC II. This interaction is necessary for isotype switching (to IgG, IgA, IgE) and the formation of memory B-cells. * **Options C & D (MHC I/II Dependent):** MHC molecules can only bind and present **peptide (protein) fragments**. Since polysaccharides are not proteins, they cannot be loaded onto MHC I or MHC II molecules. Therefore, they do not trigger a classical T-cell mediated immune response. **3. High-Yield Clinical Pearls for NEET-PG:** * **Age Factor:** TI antigens are poorly immunogenic in children under **2 years of age** because their splenic marginal zone (where B-cells respond to TI antigens) is immature. * **Vaccine Strategy:** To make polysaccharide vaccines (like the Pneumococcal vaccine) effective in infants, they are **conjugated** to a protein carrier (e.g., diphtheria toxoid). This converts the TI antigen into a T-cell dependent one, allowing for isotype switching and long-term memory. * **Memory:** Pure polysaccharide antigens do **not** produce immunological memory or secondary (anamnestic) responses.

Question 434: All of the following are functions of CD4 helper cells, except?

A. Immunogenic memory
B. Produce immunoglobulins (Correct Answer)
C. Activate macrophages
D. Activate cytotoxic cells

Explanation: **Explanation:** The correct answer is **B. Produce immunoglobulins.** In immunology, it is a fundamental concept that **B-lymphocytes** (specifically plasma cells) are the only cells capable of producing and secreting immunoglobulins (antibodies). CD4+ T-helper cells do not produce antibodies themselves; instead, they act as the "conductors" of the immune system by secreting cytokines that signal other cells to perform their functions. **Analysis of Options:** * **A. Immunogenic memory:** CD4 cells can differentiate into **Memory T-cells** after an initial encounter with an antigen. These cells persist long-term and provide a rapid, heightened response upon re-exposure. * **C. Activate macrophages:** Th1 cells (a subset of CD4 cells) secrete **Interferon-gamma (IFN-γ)**, which is the potent activator of macrophages, enhancing their phagocytic and microbicidal activity. * **D. Activate cytotoxic cells:** CD4 cells provide "help" to CD8+ T-cells through the secretion of **IL-2**, which is essential for the proliferation and differentiation of cytotoxic T-lymphocytes (CTLs). **High-Yield Clinical Pearls for NEET-PG:** * **MHC Restriction:** CD4 cells recognize antigens presented on **MHC Class II** molecules (found on Antigen Presenting Cells), while CD8 cells recognize **MHC Class I**. * **Th1 vs. Th2:** Th1 cells primarily mediate cellular immunity (IFN-γ, IL-2), whereas Th2 cells mediate humoral immunity by secreting **IL-4, IL-5, and IL-13** to stimulate B-cells to undergo class switching and antibody production. * **HIV Pathogenesis:** The hallmark of HIV/AIDS is the progressive depletion of **CD4+ T-cells**, leading to a collapse of both cell-mediated and humoral immune coordination.

Question 435: The prozone phenomenon is responsible for which of the following?

A. False negative test (Correct Answer)
B. False positive test
C. May cause any of the above
D. Has no relation with the accuracy of the test

Explanation: ### Explanation The **Prozone Phenomenon** is a classic immunological concept frequently tested in NEET-PG. It occurs when there is an **excess of antibodies** in a patient’s serum relative to the amount of antigen present in the test system. #### 1. Why the Correct Answer is Right (False Negative Test) For a visible agglutination or precipitation reaction to occur, antigens and antibodies must be in an **optimal proportion** (the Zone of Equivalence) to form a large, stable lattice. * In the **Prozone**, the high concentration of antibodies results in each antigenic determinant being saturated by a single antibody molecule. * This prevents the "cross-linking" required to form a lattice. * As a result, no visible reaction occurs, leading to a **False Negative** result despite the presence of specific antibodies. This is corrected by **diluting the serum**. #### 2. Why Other Options are Wrong * **False Positive (B):** A false positive occurs due to cross-reactivity or non-specific binding. The prozone phenomenon specifically inhibits a reaction that *should* have happened; it does not create a reaction where none exists. * **May cause any of the above (C) / No relation (D):** These are incorrect because the prozone phenomenon has a specific, predictable impact on diagnostic accuracy—it exclusively leads to under-detection (false negatives). #### 3. Clinical Pearls & High-Yield Facts * **Postzone Phenomenon:** This occurs due to an **excess of antigen**. Like the prozone, it also results in a false negative because the lattice cannot form. * **Clinical Significance:** The prozone phenomenon is most commonly encountered in: 1. **Syphilis testing (VDRL/RPR):** Especially in secondary syphilis where antibody titers are very high. 2. **Brucellosis (Standard Agglutination Test):** Often requires serial dilutions to bypass the prozone. * **NEET-PG Tip:** If a question mentions a "negative test result that becomes positive upon dilution," the answer is always the **Prozone Phenomenon**.

Question 436: Which of the following is NOT an Antigen Presenting Cell (APC)?

A. Langerhan's cell
B. Dendritic cells
C. T-cells (Correct Answer)
D. B-cells

Explanation: ### Explanation Antigen-Presenting Cells (APCs) are specialized cells that capture, process, and display antigens on their surface via **MHC Class II molecules** to activate T-lymphocytes. **Why T-cells are the correct answer:** T-cells are the **recipients** of the antigen signal, not the presenters. They possess T-cell receptors (TCRs) that recognize antigens presented by APCs. While T-cells are central to the adaptive immune response, they do not express MHC Class II molecules (unless activated in certain contexts) and do not function to initiate the immune response by presenting exogenous antigens to other cells. **Why the other options are incorrect:** * **Dendritic cells (B):** These are the most potent "Professional APCs." They are the only cells capable of activating naive T-cells. * **Langerhans cells (A):** These are specialized dendritic cells found in the **stratum spinosum** of the epidermis. They capture skin antigens and migrate to local lymph nodes to present them. * **B-cells (D):** These are professional APCs that internalize antigens via receptor-mediated endocytosis to present them to Helper T-cells (CD4+), which in turn triggers B-cell differentiation into plasma cells. **High-Yield Facts for NEET-PG:** * **Professional APCs:** Dendritic cells, Macrophages, and B-cells. (Mnemonic: **B**e **M**y **D**og). * **MHC Restriction:** APCs present antigens via **MHC Class II** to **CD4+ T-cells**. All nucleated cells present endogenous antigens via **MHC Class I** to **CD8+ T-cells**. * **Follicular Dendritic Cells (FDCs):** Found in germinal centers; unlike regular dendritic cells, they lack MHC II and present trapped antigens to B-cells. * **Co-stimulation:** For full T-cell activation, APCs must provide a second signal (e.g., **B7** on APC binding to **CD28** on T-cell).

Question 437: What is a possible source of a "second signal" to a B-cell bound by a specific antigen?

A. Epstein-Barr virus
B. Endotoxin
C. Antigen-specific T-cells (Correct Answer)
D. Plasma cells

Explanation: To activate a B-cell and initiate an antibody response, two distinct signals are typically required. This is a high-yield concept in immunology known as the **Two-Signal Hypothesis**. ### **Explanation of the Correct Answer** * **Signal 1:** Occurs when the B-cell receptor (BCR) binds to its specific antigen. * **Signal 2 (The "Second Signal"):** For T-dependent antigens (mostly proteins), the B-cell processes the antigen and presents it via **MHC-II** to a **Helper T-cell (CD4+)**. The interaction between **CD40** on the B-cell and **CD40L** on the T-cell provides the essential second signal. This leads to B-cell proliferation, isotype switching, and memory cell formation. ### **Analysis of Incorrect Options** * **A. Epstein-Barr Virus (EBV):** While EBV can activate B-cells by binding to the CD21 receptor, it is a mechanism of polyclonal activation/transformation rather than a physiological "second signal" for a specific antigen-bound B-cell. * **B. Endotoxin (LPS):** Endotoxins act as **T-independent antigens**. At high concentrations, they can cause polyclonal B-cell activation via Toll-like receptors (TLR-4), bypassing the need for a specific T-cell "second signal," but they do not provide the classical second signal for specific antibody production. * **C. Plasma Cells:** These are the end-stage effector cells of the B-cell lineage that secrete antibodies. They do not provide activation signals to other B-cells. ### **High-Yield Clinical Pearls for NEET-PG** * **Hyper-IgM Syndrome:** Caused by a deficiency in **CD40L** on T-cells. Without this "second signal," B-cells cannot undergo class switching, leading to high IgM but low IgG, IgA, and IgE. * **T-Independent Antigens:** Polysaccharides (like those in the Pneumococcal capsule) do not involve T-cells. They provide a "second signal" through extensive cross-linking of BCRs or TLR stimulation, but they **do not** produce immunological memory. * **Hapten-Carrier Effect:** A hapten (small molecule) becomes immunogenic only when linked to a protein carrier, which allows for T-cell involvement (the second signal).

Question 438: What role does the lipopolysaccharide of Gram-negative bacteria play in the immune response?

A. Hapten
B. Heterophile antibody
C. Stimulator for B lymphocytes (Correct Answer)
D. Inducer of cell-mediated immunity

Explanation: **Explanation:** The correct answer is **C. Stimulator for B lymphocytes.** Lipopolysaccharide (LPS), found in the outer membrane of Gram-negative bacteria, acts as a classic **T-cell independent (TI) antigen**. Specifically, it is a **polyclonal B-cell activator** (mitogen). Unlike T-dependent antigens that require helper T-cells to trigger an immune response, LPS can directly cross-link B-cell receptors or bind to **Toll-like Receptor 4 (TLR4)**. This interaction stimulates B-cells to proliferate and secrete antibodies (primarily IgM) without the need for T-cell mediation. **Analysis of Incorrect Options:** * **A. Hapten:** A hapten is a small molecule that is antigenic but not immunogenic on its own; it requires a carrier protein to elicit an immune response. LPS is a large, complex molecule that is highly immunogenic by itself. * **B. Heterophile antibody:** These are antibodies that react with antigens from phylogenetically unrelated species (e.g., Paul-Bunnell test). LPS is an *antigen*, not an antibody. * **D. Inducer of cell-mediated immunity (CMI):** CMI is primarily driven by T-lymphocytes and intracellular pathogens. As a TI-antigen, LPS predominantly triggers a humoral (antibody) response rather than a T-cell-mediated one. **High-Yield NEET-PG Pearls:** * **Endotoxin Structure:** LPS consists of three parts: Lipid A (responsible for **toxicity** and cytokine storm), Core polysaccharide, and O-antigen (responsible for **serological specificity**). * **Cytokine Release:** LPS triggers macrophages to release **IL-1, IL-6, and TNF-α**, which can lead to septic shock and DIC. * **Schwartzman Reaction:** This is a local or systemic reaction to repeated injections of LPS, characterized by hemorrhagic necrosis.

Question 439: Delayed hypersensitivity in skin tests is assessed by which of the following findings?

A. Erythema
B. Bulla
C. Necrosis
D. Induration (Correct Answer)

Explanation: **Explanation:** **Induration** is the hallmark of a positive Type IV (Delayed-type) hypersensitivity reaction. Unlike immediate hypersensitivity (Type I), which is mediated by IgE and histamine, Type IV reactions are **cell-mediated**. 1. **Why Induration is Correct:** When an antigen (e.g., Tuberculin/PPD) is injected intradermally, sensitized **T-lymphocytes** (Th1 cells) migrate to the site. They release cytokines (IFN-γ, IL-2) that recruit and activate **macrophages**. This massive cellular infiltration and the resulting deposition of fibrin in the local tissue cause the area to become firm and raised—a process known as **induration**. This reaction typically peaks at **48–72 hours**. 2. **Why Other Options are Incorrect:** * **Erythema (A):** While redness often accompanies the reaction due to vasodilation, it is non-specific and can occur in Type I reactions or simple irritation. It is **not** used to measure the strength of a delayed hypersensitivity test. * **Bulla (B) and Necrosis (C):** These represent extreme, severe tissue damage. While they can occur in hyper-responsive individuals (e.g., a strongly positive Mantoux test), they are complications rather than the standard diagnostic criteria for assessing the reaction. **NEET-PG High-Yield Pearls:** * **Classic Examples:** Mantoux test (Tuberculosis), Lepromin test (Leprosy), Frei test (Lymphogranuloma Venereum), and Contact Dermatitis. * **Key Cells:** CD4+ T-cells (Th1) and Macrophages. * **Measurement:** In a Mantoux test, only the **transverse diameter of the induration** is measured in millimeters, not the erythema. * **Time Frame:** "Delayed" refers to the 48–72 hour window required for T-cell recruitment and cytokine production.

Question 440: A 10-month-old patient presents with recurrent pyogenic infections, eczema, and severe bleeding due to thrombocytopenia. The diagnosis is Wiskott-Aldrich syndrome. This immune disorder is primarily associated with a deficiency in which components of the immune system?

A. Normal humoral and normal cellular immunity
B. Normal humoral immunity and deficient cellular immunity
C. Deficient humoral immunity and normal cellular immunity
D. Deficient humoral immunity and deficient cellular immunity (Correct Answer)

Explanation: **Explanation:** **Wiskott-Aldrich Syndrome (WAS)** is an X-linked recessive primary immunodeficiency caused by a mutation in the **WASP gene**, which encodes the Wiskott-Aldrich Syndrome Protein. This protein is crucial for actin cytoskeleton remodeling in hematopoietic cells. **1. Why Option D is correct:** The defect in actin polymerization impairs the ability of immune cells to migrate, interact, and signal effectively. * **Cellular Immunity:** T-cell function and numbers progressively decline (lymphopenia), leading to impaired delayed-type hypersensitivity. * **Humoral Immunity:** There is a poor antibody response to polysaccharide antigens. Characteristically, serum **IgM levels are low**, while IgA and IgE levels are often elevated (IgG is usually normal). Because both arms of the immune system are compromised, it is classified as a **combined immunodeficiency**. **2. Why other options are incorrect:** * **Options A, B, and C:** These are incorrect because WAS is not an isolated deficiency. The clinical triad of **Infections** (immunodeficiency), **Eczema**, and **Thrombocytopenia** (micro-platelets) reflects a systemic failure of both T-cell mediated and B-cell mediated (antibody) responses. **3. Clinical Pearls for NEET-PG:** * **Mnemonic (WATER):** **W**iskott-**A**ldrich, **T**hrombocytopenia, **E**czema, **R**ecurrent infections. * **Platelet Morphology:** It is the only condition featuring **small platelets** (low Mean Platelet Volume). * **Inheritance:** X-linked Recessive (affects males). * **Malignancy Risk:** Patients have a high predisposition to Non-Hodgkin Lymphoma and autoimmune diseases. * **Definitive Treatment:** Hematopoietic stem cell transplant (HSCT).

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Immunology — MCQs

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