Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 401: Which of the following is true about anaphylaxis?

A. It is mediated by allergen-specific IgE
B. Type-I hypersensitivity reaction
C. Cytokines like IL4, IL5 and IL6 along with histamine are released
D. All of the above (Correct Answer)

Explanation: Anaphylaxis is a classic example of a **Type I Hypersensitivity reaction**, which is an immediate, systemic, and potentially life-threatening allergic response. ### **Explanation of Options:** * **Option A (IgE Mediated):** The process begins with "sensitization," where an allergen triggers B-cells to produce **allergen-specific IgE antibodies**. These antibodies bind to high-affinity receptors (FcεRI) on the surface of mast cells and basophils. Upon re-exposure, the allergen cross-links these IgE molecules, triggering degranulation. * **Option B (Type-I Hypersensitivity):** By definition, anaphylaxis is the systemic form of Type I (Immediate) hypersensitivity. It occurs within minutes of exposure to an antigen (e.g., bee sting, penicillin, or peanuts). * **Option C (Mediators):** Degranulation releases pre-formed mediators like **histamine** (causing vasodilation and bronchospasm). Simultaneously, Th2 cells release cytokines: **IL-4** (induces IgE switching), **IL-5** (activates eosinophils), and **IL-13** (mucus production). Since all statements accurately describe the pathophysiology of anaphylaxis, **Option D is the correct answer.** ### **NEET-PG High-Yield Pearls:** * **Primary Mediator:** Histamine is the chief mediator, but **Tryptase** is the most specific marker used to confirm a clinical diagnosis of anaphylaxis post-event. * **Drug of Choice:** **Adrenaline (Epinephrine)** 1:1000 concentration given **Intramuscularly (IM)** in the anterolateral thigh. * **Late-phase reaction:** Occurs 2–8 hours later, primarily mediated by eosinophils and Th2 cytokines, even if the initial trigger is gone. * **Shock Type:** Anaphylaxis leads to **Distributive Shock** due to massive peripheral vasodilation.

Question 402: Which of the following is a secondary lymphoid organ?

A. Liver
B. Spleen (Correct Answer)
C. Bone marrow
D. Thymus

Explanation: ### Explanation Lymphoid organs are categorized into two types based on their function in lymphocyte development and the immune response: **1. Why the Correct Answer is Right:** **Option B (Spleen)** is a **Secondary Lymphoid Organ (SLO)**. Secondary lymphoid organs are the sites where mature lymphocytes (T and B cells) migrate, encounter antigens, and initiate an adaptive immune response. The spleen specifically filters blood-borne antigens and is the primary site for immune responses against systemic infections. Other SLOs include lymph nodes, MALT (Mucosa-Associated Lymphoid Tissue), and Peyer’s patches. **2. Why the Incorrect Options are Wrong:** * **Option C (Bone Marrow) & Option D (Thymus):** These are **Primary Lymphoid Organs**. These are the sites of **lymphopoiesis**, where lymphoid stem cells differentiate and mature into antigen-sensitive cells. B-cells mature in the bone marrow, while T-cells mature in the thymus. * **Option A (Liver):** While the liver contains immunological cells (Kupffer cells) and is a major site of hematopoiesis in the fetus, it is not classified as a primary or secondary lymphoid organ in the adult immune system. **3. NEET-PG High-Yield Pearls:** * **Primary vs. Secondary:** Primary = Production/Maturation; Secondary = Proliferation/Response. * **The Spleen:** It is the largest lymphoid organ in the body. The **White Pulp** of the spleen is the area responsible for immunological functions. * **Thymic Involution:** The thymus is most active during childhood and undergoes atrophy (involution) after puberty, being replaced by fat. * **Bone Marrow:** It serves a dual role—it is a primary lymphoid organ (B-cell maturation) and also functions as a secondary lymphoid organ for memory B-cells.

Question 403: What is the antigen combining site of the antibody called?

A. Idiotype
B. Paratope (Correct Answer)
C. Epitope
D. Hapten

Explanation: ### Explanation **Correct Answer: B. Paratope** The **paratope** is the specific region of an antibody molecule that recognizes and binds to an antigen. It is located at the amino-terminal end of the antibody molecule and is formed by the **Hypervariable Regions** (also known as Complementarity Determining Regions or CDRs) of both the heavy and light chains. Think of the paratope as the "lock" on the antibody that fits the specific "key" of the antigen. **Analysis of Incorrect Options:** * **A. Idiotype:** This refers to the unique set of antigenic determinants (idiotopes) located in the V region of an antibody. It characterizes a specific clone of B-cells. Essentially, the idiotype is what makes one antibody's binding site different from another's. * **C. Epitope:** Also known as the **antigenic determinant**, this is the specific part of the **antigen** to which the antibody binds. The paratope (on the antibody) binds to the epitope (on the antigen). * **D. Hapten:** These are small, non-immunogenic molecules that cannot elicit an immune response on their own. However, when conjugated to a larger **carrier protein**, they become immunogenic and can react with specific antibodies. **NEET-PG High-Yield Pearls:** * **Valency:** A basic IgG molecule has a valency of 2, meaning it has two paratopes and can bind two epitopes. * **Hypervariable Regions:** There are 3 CDRs on the light chain and 3 on the heavy chain; these 6 loops together form the paratope. * **Affinity vs. Avidity:** **Affinity** is the strength of binding between a single paratope and an epitope, while **Avidity** is the overall cumulative strength of binding between a multivalent antibody (like IgM) and an antigen. * **Papain Digestion:** Cleaves the antibody into two **Fab** fragments (which contain the paratope) and one **Fc** fragment.

Question 404: All of the following belong to the group of microbial sensors, EXCEPT?

A. Toll-like receptors (TLRs)
B. NOD-like receptors (NLRs)
C. Melanoma differentiation-associated gene 5 (MDA-5)
D. None of the above (Correct Answer)

Explanation: ### Explanation The question asks to identify which of the listed options is **not** a microbial sensor. Microbial sensors, collectively known as **Pattern Recognition Receptors (PRRs)**, are essential components of the innate immune system. They detect conserved molecular structures known as Pathogen-Associated Molecular Patterns (PAMPs). **Why "None of the above" is correct:** All three options (TLRs, NLRs, and MDA-5) are well-established classes of PRRs. Since every listed option functions as a microbial sensor, none of them can be excluded. **Analysis of Options:** * **Toll-like Receptors (TLRs):** These are the most well-characterized PRRs. They are transmembrane proteins located either on the cell surface (e.g., TLR-4 for LPS) or in endosomes (e.g., TLR-3, 7, 8, 9 for nucleic acids). * **NOD-like Receptors (NLRs):** These are cytosolic receptors. **NOD1 and NOD2** sense bacterial peptidoglycans, while other NLRs (like NLRP3) are involved in forming the **inflammasome**, which activates Interleukin-1β. * **MDA-5 (Melanoma Differentiation-Associated gene 5):** This belongs to the **RIG-I-like receptor (RLR)** family. It is a cytosolic sensor that specifically detects viral double-stranded RNA (dsRNA), triggering the production of Type I Interferons. **High-Yield Clinical Pearls for NEET-PG:** * **TLR-4** is the specific sensor for **Lipopolysaccharide (LPS)** of Gram-negative bacteria. * **TLR-3** senses dsRNA; **TLR-5** senses Flagellin; **TLR-9** senses unmethylated CpG DNA. * **Mutations in NOD2** are strongly associated with **Crohn’s Disease**. * **Location matters:** TLRs 1, 2, 4, 5, 6 are on the **plasma membrane**; TLRs 3, 7, 8, 9 are **intracellular (endosomal)**. NLRs and RLRs are always **cytosolic**.

Question 405: The HLA class-III region genes are important elements in which of the following?

A. Transplant rejection phenomenon
B. Complement system (Correct Answer)
C. Immune surveillance
D. Antigen presentation and elimination

Explanation: The Human Leukocyte Antigen (HLA) complex is located on the short arm of **Chromosome 6**. While Class I and II genes are primarily involved in antigen recognition, the **Class III region** is functionally distinct. ### **Why "Complement System" is Correct** The HLA Class III region does not encode for cell-surface molecules involved in antigen presentation. Instead, it encodes several secreted proteins involved in the innate immune response and inflammation. Specifically, it contains genes for: * **Complement components:** C2, C4 (C4A and C4B), and Factor B of the alternative pathway. * **Cytokines:** Tumor Necrosis Factor (TNF-α and TNF-β). * **Heat Shock Proteins (HSP).** ### **Why Other Options are Incorrect** * **A & D (Transplant rejection / Antigen presentation):** These are functions of **HLA Class I** (HLA-A, B, C) and **Class II** (HLA-DR, DQ, DP). Class I molecules present endogenous antigens to CD8+ T-cells, while Class II molecules present exogenous antigens to CD4+ T-cells. Mismatch in these classes is the primary driver of graft rejection. * **C (Immune surveillance):** This is primarily mediated by Natural Killer (NK) cells and T-lymphocytes through the recognition of HLA Class I molecules. ### **High-Yield Clinical Pearls for NEET-PG** * **Gene Locus:** HLA Class III is located *between* the Class I and Class II loci on Chromosome 6. * **Disease Association:** Deficiencies in C4 (encoded in Class III) are strongly associated with **Systemic Lupus Erythematosus (SLE)**. * **Class I vs. II Structure:** Class I has one heavy chain and a $\beta_2$-microglobulin (Chrom. 15); Class II has two polypeptide chains ($\alpha$ and $\beta$), both encoded within the MHC locus.

Question 406: A 19-year-old college student who works part-time in a pediatric AIDS clinic develops a rash. If her blood is drawn and tested for specific antibody to the chickenpox virus (varicella-zoster), which of the following antibody classes would you expect to find if she is immune to chickenpox?

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

Explanation: **Explanation:** The question tests the understanding of immunoglobulin kinetics and the markers of long-term immunity. **Why IgG is the correct answer:** **IgG** is the most abundant immunoglobulin in the serum and is the primary antibody responsible for **long-term immunity** following an infection or vaccination. In the context of Varicella-Zoster Virus (VZV), once the primary infection (chickenpox) resolves, the body maintains memory B cells that produce IgG. If a person is "immune," it implies they have pre-existing protective antibodies (IgG) that can neutralize the virus upon re-exposure. **Why the other options are incorrect:** * **IgA:** This is the primary secretory antibody found in mucous membranes (tears, saliva, colostrum). While it provides local mucosal defense, it is not the standard systemic marker used to define clinical immunity to chickenpox. * **IgM:** This is the first antibody produced during an **acute/primary infection**. Its presence indicates a current or very recent infection, not long-term immunity. * **IgD:** This is primarily found on the surface of B cells as an antigen receptor; it has no significant role in protective immunity or diagnostic serology for VZV. **NEET-PG High-Yield Pearls:** * **IgG:** The only antibody that crosses the placenta (provides passive immunity to the fetus). It has the longest half-life (~23 days). * **IgM:** The largest antibody (pentamer) and the best at complement fixation. It does not cross the placenta. * **VZV Serology:** In a clinical setting, an **IgG ELISA** is the standard test to screen healthcare workers for immunity. If IgG is negative, the individual is susceptible and requires vaccination. * **Tzanck Smear:** Used for rapid diagnosis of VZV/HSV, showing **multinucleated giant cells** with Cowdry type A inclusion bodies.

Question 407: The Complement Fixation test is used for the diagnosis of which condition?

A. Typhoid fever
B. Hemolytic disease of the newborn
C. Syphilis (Correct Answer)
D. Syphilis

Explanation: **Explanation:** The **Complement Fixation Test (CFT)** is a classical serological method based on the principle that if an antigen-antibody reaction occurs, the available complement in the serum is "fixed" (consumed). If no reaction occurs, the complement remains free to lyse indicator system cells (sheep RBCs coated with amboceptor). **Why Syphilis is correct:** The most famous application of CFT is the **Wassermann Reaction**, used for the diagnosis of Syphilis. In this test, the patient's serum is mixed with the cardiolipin antigen and a known amount of complement. If the patient has syphilis antibodies (reagin), the complement is fixed. When the indicator system is added, **no hemolysis** occurs, signifying a **positive** result. **Analysis of Incorrect Options:** * **Typhoid fever:** Diagnosed primarily via the **Widal test** (an agglutination test) or blood/stool/urine cultures. * **Hemolytic disease of the newborn (HDN):** Diagnosed using the **Direct Coombs Test** (Antiglobulin test) to detect antibodies bound to the surface of fetal red blood cells. **NEET-PG High-Yield Pearls:** * **Indicator System:** Consists of Sheep RBCs + Rabbit Antiserum to sheep RBCs (Amboceptor). * **Positive Result:** No Hemolysis (Complement was fixed by Ag-Ab complex). * **Negative Result:** Hemolysis (Complement was free to lyse the indicator RBCs). * **Modern usage:** While largely replaced by ELISA and Treponemal tests (like TPHA/FTA-ABS) due to complexity, CFT remains high-yield for exams regarding its historical association with the Wassermann test and certain viral/fungal infections (e.g., Histoplasmosis).

Question 408: Which of the following is NOT true of carbohydrate antigens?

A. Poorly immunogenic
B. Often elicit a T-cell independent response
C. Can stimulate antibody production (Correct Answer)
D. Typically T-cell dependent responses

Explanation: ### Explanation The question asks for the statement that is **NOT true** regarding carbohydrate antigens. The correct answer is **D (Typically T-cell dependent responses)**, as carbohydrate antigens are classically **T-cell independent**. #### 1. Why Option D is the Correct Choice (The "False" Statement) Most carbohydrate antigens (like bacterial capsular polysaccharides) are **T-cell independent (TI) antigens**. Unlike proteins, they cannot be processed and presented on MHC molecules to T-cells. Therefore, they stimulate B-cells directly without T-helper cell involvement. This results in a response characterized by a lack of memory cells, no isotype switching (mostly IgM), and poor immunogenicity in infants. #### 2. Analysis of Other Options (True Statements) * **A. Poorly immunogenic:** True. Because they lack T-cell involvement, the immune response is weaker compared to proteins. They are particularly non-immunogenic in children under 2 years of age due to immature splenic function. * **B. Often elicit a T-cell independent response:** True. As explained above, their repetitive structures cross-link B-cell receptors (BCR) directly. * **C. Can stimulate antibody production:** True. While the response is limited, they do stimulate B-cells to produce antibodies (primarily IgM). #### 3. Clinical Pearls for NEET-PG * **Conjugate Vaccines:** To make carbohydrate antigens T-cell dependent (and thus more effective), they are conjugated to a protein carrier (e.g., **Hib vaccine**, **PCV13**). This allows T-cells to recognize the protein component and provide "help" to B-cells. * **Hapten vs. Antigen:** A carbohydrate can act as a hapten; it is antigenic (can react with antibodies) but requires a carrier to be fully immunogenic. * **Zwitterionic Polysaccharides:** A rare exception; some carbohydrates with both positive and negative charges (e.g., *B. fragilis* capsule) **can** be presented to T-cells.

Question 409: Which molecule activates macrophages via the classical pathway?

A. IFN-gamma (Correct Answer)
B. IL-4
C. IL-13
D. IL-1

Explanation: **Explanation:** The activation of macrophages is a critical component of the cell-mediated immune response and occurs via two distinct pathways: the **Classical (M1)** and the **Alternative (M2)** pathways. **Why IFN-gamma is correct:** The **Classical Pathway (M1)** is primarily triggered by **Interferon-gamma (IFN-γ)**, which is secreted by Th1 cells and Natural Killer (NK) cells. When macrophages are activated via this pathway, they produce reactive oxygen species (ROS), nitric oxide (NO), and lysosomal enzymes. These M1 macrophages are highly pro-inflammatory and are specialized for killing intracellular pathogens (like *M. tuberculosis*) and tumor cells. **Why the other options are incorrect:** * **IL-4 and IL-13 (Options B & C):** These cytokines trigger the **Alternative Pathway (M2)**. M2 macrophages are involved in tissue repair, wound healing, and anti-inflammatory responses. They are not microbicidal in the same way as M1 cells. * **IL-1 (Option D):** This is a pro-inflammatory cytokine produced *by* activated macrophages (and other cells) to induce fever and acute-phase responses, but it is not the primary inducer of classical macrophage activation. **High-Yield NEET-PG Pearls:** * **M1 Activation:** Induced by IFN-γ and microbial products (LPS). Function: Microbicidal, pro-inflammatory. * **M2 Activation:** Induced by IL-4 and IL-13. Function: Tissue repair, fibrosis, anti-inflammatory (IL-10 secretion). * **Granuloma Formation:** IFN-γ is the key cytokine that transforms macrophages into **epithelioid cells** in granulomatous diseases like Tuberculosis. * **Memory Aid:** **M1** = **M**urder (pro-inflammatory/killing); **M2** = **M**end (repair/healing).

Question 410: Which of the following is the most potent antigen-presenting cell?

A. Fibroblasts
B. Dendritic cells (Correct Answer)
C. Macrophages
D. B Cells

Explanation: **Explanation:** Antigen-presenting cells (APCs) are specialized cells that capture antigens, process them into peptides, and present them via MHC molecules to T cells. While several cells can perform this function, **Dendritic Cells (DCs)** are considered the most potent and efficient APCs. **Why Dendritic Cells are the correct answer:** Dendritic cells are the only APCs capable of activating **naive T cells**, making them the primary initiators of the adaptive immune response. They possess high levels of MHC Class II molecules and potent co-stimulatory signals (B7-1 and B7-2). Their unique ability to perform **cross-presentation** (presenting exogenous antigens on MHC Class I to CD8+ T cells) further cements their status as the "professional" APC. **Analysis of Incorrect Options:** * **Macrophages (Option C):** While they are professional APCs, their primary role is phagocytosis and killing of microbes. They present antigens to **already activated** effector T cells to enhance their own microbicidal activity. * **B Cells (Option D):** These are professional APCs that present antigens to Helper T cells (CD4+) to receive signals for antibody production. They are efficient at capturing soluble antigens via surface immunoglobulins but are less potent than DCs in initiating primary responses. * **Fibroblasts (Option A):** These are "non-professional" or "atypical" APCs. They can express MHC Class II only under specific inflammatory conditions (e.g., stimulation by IFN-gamma) but lack the constitutive machinery of professional APCs. **NEET-PG High-Yield Pearls:** * **Langerhans cells** are specialized dendritic cells found in the epidermis (contain Birbeck granules). * **Follicular Dendritic Cells (FDCs)** are found in germinal centers; unlike regular DCs, they lack MHC II and trap antigens in the form of immune complexes for B cell memory. * **Professional APCs** include Dendritic cells, Macrophages, and B cells.

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Immunology — MCQs

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