Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 601: Graft versus host reaction is mediated by which of the following?

A. Macrophages
B. T lymphocytes (Correct Answer)
C. B lymphocytes
D. T lymphocytes and macrophages

Explanation: **Explanation:** **Graft-versus-Host Disease (GVHD)** occurs when immunologically competent cells (the graft) recognize the recipient’s (the host) tissues as foreign and mount an immune attack against them. **Why T lymphocytes are correct:** The primary mediators of GVHD are **mature donor T lymphocytes**. For GVHD to occur, three conditions (Billingham’s criteria) must be met: 1. The graft must contain immunologically competent cells (specifically T cells). 2. The recipient must possess antigens lacking in the donor (making the host appear "foreign"). 3. The recipient must be immunocompromised or unable to reject the donor cells. When donor T cells (both CD4+ and CD8+) are infused, they recognize the host’s Major Histocompatibility Complex (MHC) antigens, undergo activation, and cause direct cytotoxic damage and cytokine-mediated inflammation. **Why other options are incorrect:** * **A & D (Macrophages):** While macrophages are involved in the effector phase of inflammation and tissue damage, they are not the primary mediators or initiators of the reaction. GVHD is fundamentally a T-cell-driven process. * **C (B lymphocytes):** B cells produce antibodies. While they may play a minor role in chronic GVHD, the classic graft-versus-host reaction is a **Type IV (Cell-Mediated) Hypersensitivity** response, which is the hallmark of T-cell activity. **High-Yield Clinical Pearls for NEET-PG:** * **Common Sites:** GVHD typically affects the **skin** (rash), **liver** (jaundice/elevated enzymes), and **GIT** (diarrhea). * **Common Scenarios:** Most frequently seen in **Allogeneic Bone Marrow Transplants**. It can also occur after blood transfusions in severely immunocompromised patients. * **Prevention:** To prevent transfusion-associated GVHD, blood products should be **irradiated** to inactivate donor T lymphocytes. * **Acute vs. Chronic:** Acute GVHD occurs within 100 days; Chronic GVHD occurs after 100 days.

Question 602: Division and differentiation of B cells leading to production of plasma cells both require:

A. CD4 and CD8
B. Interleukin (IL)-1 and IL-5
C. IL-5 only
D. IL-4 and IL-6 (Correct Answer)

Explanation: The production of antibodies is a multi-step process involving the activation, proliferation (division), and differentiation of B cells into plasma cells. This process is primarily driven by cytokines secreted by **Th2 (T-helper 2) cells**. ### Why IL-4 and IL-6 is the Correct Answer * **IL-4 (The B-cell Growth Factor):** It is the primary cytokine responsible for the initial activation and **proliferation (division)** of B cells. It also plays a crucial role in "Class Switching" (e.g., switching from IgM to IgE). * **IL-6 (The B-cell Differentiation Factor):** It acts at the later stages of the B-cell response, stimulating the **differentiation** of activated B cells into mature, antibody-secreting **plasma cells**. * **Synergy:** Together, these cytokines ensure the expansion of the B-cell pool and their subsequent maturation into functional effector cells. ### Explanation of Incorrect Options * **A. CD4 and CD8:** These are surface markers for T-helper and T-cytotoxic cells, respectively. While CD4+ T cells help B cells, CD8+ cells are involved in cell-mediated cytotoxicity, not B-cell differentiation. * **B. IL-1 and IL-5:** IL-1 is a pro-inflammatory cytokine (endogenous pyrogen). While **IL-5** does aid in B-cell growth and IgA production, IL-1 is not a primary driver of B-cell division/differentiation. * **C. IL-5 only:** IL-5 is specifically known as the **Eosinophil differentiation factor**. While it supports B-cell growth, it cannot fulfill the roles of both proliferation and differentiation alone as effectively as the IL-4/IL-6 combination. ### High-Yield Clinical Pearls for NEET-PG * **IL-4:** Stimulates IgE and IgG4 production; "Induces" B-cell growth. * **IL-5:** Stimulates **IgA** production and Eosinophil activation. * **IL-6:** An acute-phase reactant stimulator; essential for the final maturation of plasma cells. * **Mnemonic:** "Hot T-Bone stEAk" (IL-1: Fever; IL-2: T-cells; IL-3: Bone marrow; IL-4: IgE; IL-5: IgA).

Question 603: Which of the following statements about Polymerase Chain Reaction (PCR) is false?

A. PCR utilizes a heat-labile DNA polymerase. (Correct Answer)
B. PCR utilizes a heat-stable DNA polymerase.
C. PCR is a technique for DNA amplification.
D. PCR is used to yield multiple copies of DNA.

Explanation: ### Explanation **1. Why Option A is the Correct (False) Statement:** The Polymerase Chain Reaction (PCR) process involves a **denaturation step** where the reaction mixture is heated to approximately **94°C–96°C** to separate double-stranded DNA. A heat-labile (heat-sensitive) enzyme would be permanently denatured and inactivated at this temperature. Therefore, PCR requires a **heat-stable (thermostable) DNA polymerase**, most commonly **Taq polymerase** (derived from the bacterium *Thermus aquaticus*), which can withstand repeated heating cycles. **2. Analysis of Other Options:** * **Option B:** This is a true statement. Heat-stable polymerases are essential for the automation of PCR, as they remain functional throughout multiple cycles of heating and cooling. * **Options C & D:** These are true statements defining the core purpose of PCR. It is an *in vitro* enzymatic technique used for the exponential **amplification** of a specific DNA segment, yielding **millions of copies** from a single template. **3. NEET-PG Clinical Pearls & High-Yield Facts:** * **Steps of PCR:** 1. Denaturation (~95°C), 2. Annealing (~55°C), 3. Extension (~72°C). * **Components:** Template DNA, Primers (forward and reverse), dNTPs (nucleotides), and Taq Polymerase. * **RT-PCR:** Reverse Transcriptase PCR is used to amplify **RNA** (e.g., for diagnosing COVID-19 or HIV viral load) by first converting RNA into complementary DNA (cDNA). * **Real-Time PCR (qPCR):** Allows for the quantification of DNA in real-time using fluorescent dyes. * **Application:** PCR is the "gold standard" for diagnosing slow-growing or non-culturable organisms (e.g., *Mycobacterium tuberculosis*, *Chlamydia*).

Question 604: A patient presents with a painless ulcer on his penis. A Wasserman test is done and is positive. The Wasserman reaction is an example of which immunological test?

A. Complement fixation (Correct Answer)
B. Precipitation
C. Neutralization
D. Agglutination

Explanation: The **Wasserman test** is a classic serological test used for the diagnosis of syphilis. It is a prime example of a **Complement Fixation Test (CFT)**. ### Why Complement Fixation is Correct In this test, the patient’s serum is mixed with a specific antigen (cardiolipin). If antibodies are present, they form an antigen-antibody complex that "fixes" or consumes the added complement. To visualize this, an indicator system (sheep RBCs coated with hemolysin) is added. * **Positive Result:** Complement is already fixed; no lysis of RBCs occurs. * **Negative Result:** Complement is free to lyse the RBCs, resulting in hemolysis. ### Why Other Options are Incorrect * **Precipitation:** This involves the interaction of soluble antigens with antibodies to form an insoluble visible precipitate (e.g., VDRL or Kahn test). While VDRL also diagnoses syphilis, it is a flocculation test, not CFT. * **Neutralization:** This test measures the ability of an antibody to inhibit the biological activity of an antigen, such as a virus or toxin (e.g., ASO titer). * **Agglutination:** This occurs when antibodies react with particulate (insoluble) antigens, causing visible clumping (e.g., Widal test or TPHA). ### NEET-PG High-Yield Pearls * **Antigen Used:** The Wasserman test uses **Cardiolipin** (extracted from beef heart) as the antigen, making it a non-specific (non-treponemal) test. * **Modern Equivalent:** The Wasserman test is largely historical and has been replaced by the **VDRL** and **RPR** tests, which are easier to perform. * **Specific Tests:** For confirmation of syphilis, treponemal tests like **FTA-ABS** and **TPHA** are used. * **Clinical Correlation:** A painless ulcer (chancre) is the hallmark of **Primary Syphilis**.

Question 605: The human leukocyte antigen (HLA) complex is located on which chromosome?

A. 6 (Correct Answer)
B. 7
C. 8
D. 9

Explanation: **Explanation:** The **Human Leukocyte Antigen (HLA) complex**, also known as the Major Histocompatibility Complex (MHC) in humans, is a cluster of genes located on the **short arm (p) of Chromosome 6**. These genes encode surface glycoproteins that play a critical role in antigen presentation and the regulation of the immune system. * **HLA Class I (A, B, C):** Found on all nucleated cells; presents endogenous antigens to CD8+ T-cells. * **HLA Class II (DR, DQ, DP):** Found on professional antigen-presenting cells (APCs); presents exogenous antigens to CD4+ T-cells. * **HLA Class III:** Encodes components of the complement system (C2, C4) and cytokines like TNF-α. **Analysis of Incorrect Options:** * **Option B (Chromosome 7):** Associated with the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) gene and the T-cell receptor (TCR) gamma chain. * **Option C (Chromosome 8):** Contains the MYC oncogene (implicated in Burkitt lymphoma). * **Option D (Chromosome 9):** Location of the ABO blood group genes and the CDKN2A tumor suppressor gene. **High-Yield Clinical Pearls for NEET-PG:** 1. **MHC Restriction:** CD4 cells recognize Class II, and CD8 cells recognize Class I (Rule of 8: 4×2=8; 8×1=8). 2. **Disease Associations:** * **HLA-B27:** Ankylosing spondylitis, Reiter’s syndrome. * **HLA-DR3/DR4:** Type 1 Diabetes Mellitus. * **HLA-DQ2/DQ8:** Celiac disease. 3. **Inheritance:** HLA genes are codominantly expressed and inherited as a haplotype (one set from each parent).

Question 606: Which skin test is useful in the diagnosis of Hydatid disease?

A. Casoni's test (Correct Answer)
B. Schick test
C. Patch test
D. Dick's test

Explanation: **Explanation:** **Casoni’s test** is the correct answer. It is an immediate hypersensitivity (Type I) skin test used for the diagnosis of **Hydatid disease** (caused by *Echinococcus granulosus*). The test involves the intradermal injection of 0.2 ml of sterile hydatid fluid. A positive result is indicated by the formation of a large wheal (with pseudopodia) within 20 minutes. While historically significant, it is now largely replaced by serological tests (ELISA) and imaging (USG/CT) due to its low specificity and risk of anaphylaxis. **Analysis of Incorrect Options:** * **Schick test:** Used to demonstrate immunity or susceptibility to **Diphtheria** (*Corynebacterium diphtheriae*). It detects the presence of circulating antitoxin. * **Patch test:** A diagnostic tool for **Type IV (Delayed) hypersensitivity** reactions, primarily used to identify the causative agents of allergic contact dermatitis. * **Dick’s test:** Used to identify susceptibility to **Scarlet Fever**. It involves the intradermal injection of erythrogenic toxin produced by *Streptococcus pyogenes*. **High-Yield Clinical Pearls for NEET-PG:** * **Hydatid Fluid:** The antigen used in Casoni’s test is usually obtained from human or sheep hydatid cysts and sterilized by Seitz filtration. * **False Positives:** Casoni’s test can be falsely positive in cases of Taeniasis, Cirrhosis, or other helminthic infections. * **Gold Standard:** For Hydatid disease, **USG (WHO classification)** and **Serology (IgG ELISA)** are currently the preferred diagnostic modalities. * **Contraindication:** Never aspirate a hydatid cyst for diagnosis, as it may lead to fluid leakage and life-threatening anaphylaxis.

Question 607: Arthus reaction is an example of which type of hypersensitivity?

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

Explanation: ### Explanation **Correct Answer: C. Type III Hypersensitivity** The **Arthus reaction** is a classic example of **Type III (Immune-complex mediated) hypersensitivity**. It is a localized inflammatory response that occurs when an antigen is injected into the skin of an individual who already has high levels of circulating IgG antibodies. * **Mechanism:** The injected antigen reacts with pre-formed antibodies to form **insoluble immune complexes** locally. These complexes deposit in the walls of small blood vessels, leading to the activation of the **Complement system** (C5a, C3a). This recruits neutrophils, causing vasculitis, edema, and localized tissue necrosis. * **Clinical Presentation:** Typically appears within 4–10 hours after vaccination (e.g., Tetanus or Diphtheria boosters). **Why other options are incorrect:** * **Type I (Immediate):** Mediated by **IgE** and mast cell degranulation (e.g., Anaphylaxis, Asthma). * **Type II (Cytotoxic):** Mediated by **IgG or IgM** binding to antigens on cell surfaces, leading to cell lysis (e.g., Rh incompatibility, Myasthenia Gravis). * **Type IV (Delayed):** Mediated by **T-cells**, not antibodies. It takes 48–72 hours to develop (e.g., Mantoux test, Contact dermatitis). **High-Yield NEET-PG Pearls:** * **Arthus Reaction vs. Serum Sickness:** Arthus is **localized** (Type III), whereas Serum Sickness is **systemic** (Type III). * **Key Mediator:** Neutrophils and Complement (C5a). * **Mnemonic for Hypersensitivity (Gell & Coombs):** **ACID** * **A** – **A**naphylactic (Type I) * **C** – **C**ytotoxic (Type II) * **I** – **I**mmune-Complex (Type III) * **D** – **D**elayed-type (Type IV)

Question 608: Paul Bunnell reaction is a type of?

A. Agglutination (Correct Answer)
B. Complement fixation
C. Precipitation
D. Flocculation test

Explanation: The **Paul-Bunnell test** is a classic diagnostic tool used to detect **Infectious Mononucleosis** caused by the Epstein-Barr Virus (EBV). ### Why Agglutination is Correct The test is based on the principle of **Heterophile Agglutination**. Patients with EBV infection produce "heterophile antibodies" (IgM) that have the unique property of cross-reacting with antigens on the surface of red blood cells (RBCs) from different species. In this specific reaction, the patient's serum is mixed with **sheep erythrocytes**. If heterophile antibodies are present, they bind to the sheep RBCs, causing visible clumping or **agglutination**. ### Why Other Options are Incorrect * **Complement Fixation:** This involves the consumption of complement proteins during an antigen-antibody reaction. While used for other viral titers, it is not the mechanism for the Paul-Bunnell test. * **Precipitation:** This occurs when soluble antigens react with antibodies to form an insoluble precipitate. The Paul-Bunnell test uses whole cells (particulate antigen), making it an agglutination reaction, not precipitation. * **Flocculation:** This is a specific type of precipitation where the antigen-antibody complex remains suspended as "flakes" (e.g., VDRL test for Syphilis). ### High-Yield Clinical Pearls for NEET-PG * **Specificity:** The Paul-Bunnell test is not 100% specific. To differentiate EBV from serum sickness or Forssman antibodies, the **Davidsohn Differential Test** (using guinea pig kidney and beef RBCs) is performed. * **Modern Alternative:** The **Monospot test** (latex agglutination) is the faster, modern version of this reaction. * **Key Association:** Always associate "Heterophile antibodies," "Sheep RBCs," and "Agglutination" with the Paul-Bunnell test. * **Clinical Note:** This test is usually negative in "Mononucleosis-like syndrome" caused by CMV.

Question 609: Antigens processed by the exogenous antigen presentation pathway are presented in association with which of the following?

A. Fc receptors
B. IgG heavy chains
C. MHC class I molecules
D. MHC class II molecules (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. MHC class II molecules** The immune system utilizes two distinct pathways for antigen presentation based on the source of the antigen: 1. **Exogenous Pathway (MHC II):** Extracellular pathogens (bacteria, toxins) are internalized by **Antigen-Presenting Cells (APCs)** like macrophages, B cells, and dendritic cells via phagocytosis or endocytosis. These antigens are degraded in endolysosomes and then loaded onto **MHC Class II** molecules. These complexes are presented to **CD4+ T-helper cells**. 2. **Endogenous Pathway (MHC I):** Intracellular antigens (viruses, tumor proteins) are processed by proteasomes in the cytosol and loaded onto **MHC Class I** molecules in the endoplasmic reticulum. These are presented to **CD8+ Cytotoxic T cells**. #### Why Incorrect Options are Wrong: * **A. Fc receptors:** These are found on the surface of various immune cells (like NK cells and mast cells) and bind to the Fc portion of antibodies; they do not present antigens to T cells. * **B. IgG heavy chains:** These are structural components of the immunoglobulin molecule, responsible for determining the antibody class, not for antigen presentation. * **C. MHC class I molecules:** These present **endogenous** antigens (derived from within the cell) to CD8+ T cells. #### High-Yield Clinical Pearls for NEET-PG: * **Rule of 8:** MHC **II** × CD**4** = 8; MHC **I** × CD**8** = 8. * **Invariant Chain (Ii):** In the exogenous pathway, the invariant chain prevents premature binding of self-peptides to MHC II in the ER. * **Cross-presentation:** A unique process where dendritic cells can present exogenous antigens via MHC I to activate CD8+ T cells (essential for anti-tumor immunity). * **MHC II Distribution:** Unlike MHC I (found on all nucleated cells), MHC II is restricted to professional APCs.

Question 610: Which immunoglobulin is implicated in atopy and anaphylaxis?

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

Explanation: **Explanation:** **Correct Option: A (IgE)** IgE is the primary mediator of **Type I Hypersensitivity reactions**, which include atopy (allergic rhinitis, asthma, eczema) and systemic anaphylaxis. The mechanism involves the binding of IgE to high-affinity receptors (**FcεRI**) on the surface of **mast cells and basophils**. Upon re-exposure to an allergen, cross-linking of these IgE molecules triggers degranulation, releasing potent inflammatory mediators like histamine, leukotrienes, and prostaglandins. **Why other options are incorrect:** * **IgM:** This is the first antibody produced in a primary immune response. It is a pentamer and is primarily involved in complement activation and agglutination, not allergic reactions. * **IgG:** The most abundant immunoglobulin in serum. It provides long-term immunity, crosses the placenta, and mediates Type II and Type III hypersensitivity, but not the immediate Type I response. * **IgA:** Found predominantly in secretions (tears, saliva, colostrum, GI tract). Its main role is mucosal immunity by preventing the attachment of pathogens to epithelial surfaces. **High-Yield Clinical Pearls for NEET-PG:** * **Prausnitz-Küstner (PK) Reaction:** A classic test used to demonstrate the presence of IgE in serum (though now replaced by in-vitro tests like RAST). * **Heat Lability:** IgE is the most heat-labile immunoglobulin (inactivated at 56°C for 30 minutes). * **Parasitic Infections:** IgE levels also rise significantly during helminthic infestations (e.g., Ascariasis) to facilitate eosinophil-mediated killing. * **Receptor:** IgE binds to mast cells via the **Fc portion**, leaving the Fab portion free to bind allergens.

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Immunology — MCQs

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