Immunology — MCQs

A 7-month-old baby who is failing to thrive is brought into a clinic. The baby's mother died of AIDS 2 months ago. Blood is obtained and sent to the laboratory to check for HIV infection. The physician orders a test whose detection system is based on enzymatic activity. Which of the following tests is a heterogeneous immunoassay?

Q78Easy

Which of the following is an example of a CXC chemokine?

Q79Easy

Which of the following best describes Natural Killer (NK) cells?

Q80Medium

All IgG subclasses are responsible for coagglutination by binding to S. Aureus protein A, except?

Immunology Indian Medical PG Practice Questions and MCQs

Question 71: Which of the following statements is true regarding kappa, lambda, and heavy chain immunoglobulins?

A. Coded at the same site on a chromosome
B. Coded at different sites on the same chromosome
C. The chains are formed by genetic rearrangement after maturation (Correct Answer)
D. Different chains of the same immunoglobulins are coded by different chromosomes

Explanation: ### Explanation **Correct Answer: C. The chains are formed by genetic rearrangement after maturation** The diversity of immunoglobulins is achieved through a process called **Somatic Recombination (V(D)J recombination)**. In the germline state, immunoglobulin genes exist as separate segments (Variable, Diversity, Joining, and Constant). During B-cell development, these segments undergo physical rearrangement and splicing at the DNA level to form a functional gene. This process occurs as the B-cell matures from a progenitor cell to a mature B-lymphocyte, allowing for the generation of millions of different antibody specificities from a limited number of genes. **Analysis of Incorrect Options:** * **Options A & B:** These are incorrect because the genes for the three types of chains are located on **entirely different chromosomes**, not just different sites on the same chromosome. * **Option D:** While this option correctly states that different chains are on different chromosomes, it is technically less precise than Option C in the context of "how they are formed." However, in many competitive exams, the focus is on the **genetic location** (see Clinical Pearl below). **High-Yield Clinical Pearls for NEET-PG:** * **Chromosomal Locations (The "2, 22, 14" Rule):** * **Kappa (κ) Light Chain:** Chromosome **2** * **Lambda (λ) Light Chain:** Chromosome **22** * **Heavy (H) Chain:** Chromosome **14** * **Allelic Exclusion:** Only one allele (either maternal or paternal) of a heavy chain and one allele of a light chain are expressed in a single B-cell to ensure "one cell, one specificity." * **Kappa/Lambda Ratio:** In humans, the normal ratio is **2:1**. A significant shift in this ratio (clonality) is a diagnostic marker for B-cell lymphomas or Multiple Myeloma.

Question 72: B cells are induced to produce IgE by which of the following cytokines?

A. IL-2
B. IL-4 (Correct Answer)
C. IL-1
D. IL-6

Explanation: **Explanation:** The correct answer is **IL-4**. This question tests the concept of **Isotype Switching** (Class Switching), where B cells change the production of antibodies from IgM/IgD to other classes (IgG, IgA, or IgE) based on cytokine signals from T-helper cells. **Why IL-4 is correct:** IL-4 is primarily secreted by **Th2 cells**. It acts on B cells to induce the heavy-chain class switching from IgM to **IgE**. This is a critical step in Type I Hypersensitivity reactions and the immune response against helminthic infections. IL-13 often works synergistically with IL-4 to promote this IgE production. **Why the other options are incorrect:** * **IL-2:** Known as the "T-cell growth factor," it primarily stimulates the proliferation and differentiation of T-cells (CD4+ and CD8+). * **IL-1:** Produced by macrophages, it is a pro-inflammatory cytokine that acts as an endogenous pyrogen (induces fever) and activates T-cells. * **IL-6:** A pleiotropic cytokine that stimulates the synthesis of acute-phase reactants in the liver and promotes the final differentiation of B cells into plasma cells. **High-Yield Clinical Pearls for NEET-PG:** * **Th2 Pathway:** IL-4 (IgE switch), IL-5 (Eosinophil activation), and IL-13 (Mucus secretion). * **Th1 Pathway:** IFN-gamma (activates macrophages and induces IgG switching). * **TGF-beta & IL-5:** These are the primary cytokines responsible for switching B cells to produce **IgA** (important for mucosal immunity). * **Atopy:** Patients with atopic dermatitis or asthma often have an overactive Th2 response leading to elevated IL-4 and IgE levels.

Question 73: Which of the following statements is true regarding Severe Combined Immunodeficiency (SCID)?

A. Adenosine deaminase deficiency (Correct Answer)
B. Decreased circulating lymphocytes
C. NADPH oxidase deficiency
D. C1 esterase deficiency

Explanation: **Explanation:** **Severe Combined Immunodeficiency (SCID)** is a group of rare, fatal genetic disorders characterized by the profound absence of T-cell and B-cell function. 1. **Why Option A is Correct:** **Adenosine Deaminase (ADA) deficiency** is the second most common cause of SCID (autosomal recessive). ADA is an enzyme essential for purine metabolism. Its deficiency leads to the accumulation of toxic metabolites (deoxyadenosine and dATP) within lymphocytes. These metabolites are lymphotoxic, leading to the destruction of both T and B cells, resulting in "combined" immunodeficiency. 2. **Why the other options are incorrect:** * **Option B:** While SCID involves a functional loss of lymphocytes, the hallmark is the **absence or near-absence** of T-cells. "Decreased circulating lymphocytes" is too non-specific and can occur in many conditions (e.g., HIV, stress, or steroids). * **Option C:** **NADPH oxidase deficiency** is the cause of **Chronic Granulomatous Disease (CGD)**. It affects phagocyte function (neutrophils), not the adaptive immune system (T/B cells). * **Option D:** **C1 esterase inhibitor deficiency** leads to **Hereditary Angioedema**, characterized by recurrent episodes of edema, not immunodeficiency. **High-Yield Clinical Pearls for NEET-PG:** * **Most Common Cause:** X-linked SCID (due to a mutation in the **IL-2 receptor gamma chain**). * **Clinical Presentation:** Recurrent severe infections (fungal, viral, bacterial), failure to thrive, and chronic diarrhea in infancy. * **Radiology:** Absence of **thymic shadow** on chest X-ray. * **Treatment:** Hematopoietic stem cell transplant (HSCT) is the treatment of choice. ADA deficiency was the first disease treated with **gene therapy**.

Question 74: VDRL is a -

A. Slide flocculation test (Correct Answer)
B. Tube flocculation test
C. Gel precipitation test
D. Indirect haemagglutination test

Explanation: **Explanation:** The **VDRL (Venereal Disease Research Laboratory)** test is a non-specific, non-treponemal screening test for Syphilis. It detects **reagin antibodies** (IgM and IgG) produced against cardiolipin-cholesterol-lecithin antigen. 1. **Why Option A is Correct:** VDRL is a **Slide Flocculation Test**. In this reaction, the soluble antigen (cardiolipin) reacts with the patient’s serum on a slide. If antibodies are present, they form visible clumps or "floccules" that are viewed under a light microscope (10x magnification). 2. **Why Other Options are Incorrect:** * **Tube Flocculation Test:** The **Kahn test** is the classic example of a tube flocculation test for syphilis, but it is now obsolete. * **Gel Precipitation Test:** These involve diffusion in agar (e.g., Elek’s test for Diphtheria or VDRL-like reactions in gels). VDRL does not use a gel medium. * **Indirect Haemagglutination (IHA):** This involves coating RBCs with antigens. An example in syphilis is the **TPHA** (Treponema Pallidum Haemagglutination Assay), which is a specific treponemal test, not a non-specific one like VDRL. **High-Yield Clinical Pearls for NEET-PG:** * **Antigen used:** Cardiolipin (extracted from beef heart), cholesterol, and lecithin. * **Monitoring:** VDRL titers are used to **monitor the response to treatment** (titers fall after successful therapy). * **Biological False Positives (BFP):** Can occur in SLE, Leprosy, Malaria, and Pregnancy. * **Prozone Phenomenon:** Can cause a false negative result in secondary syphilis due to very high antibody titers; solved by diluting the serum. * **CSF-VDRL:** The gold standard for diagnosing **Neurosyphilis**.

Question 75: AIDS involves which of the following cells?

A. T-helper cells (Correct Answer)
B. T-suppressor cells
C. T-cytotoxic cells
D. B cells

Explanation: **Explanation:** The Human Immunodeficiency Virus (HIV), which causes AIDS, primarily targets cells expressing the **CD4 receptor** on their surface. **1. Why T-helper cells are correct:** T-helper cells (CD4+ T cells) are the primary targets of HIV. The viral envelope glycoprotein **gp120** binds specifically to the CD4 molecule. Additionally, the virus requires co-receptors (**CCR5** on macrophages/early infection or **CXCR4** on T cells/late infection) to enter the cell. Once inside, HIV replicates and eventually destroys these cells, leading to profound immunosuppression and a decline in the CD4 count, which is the hallmark of AIDS. **2. Why the other options are incorrect:** * **T-suppressor cells (CD8+):** These cells do not express the CD4 receptor. While their function is altered due to the lack of "help" from CD4 cells, they are not the primary site of viral entry or destruction. * **T-cytotoxic cells (CD8+):** Similar to suppressor cells, these lack the CD4 receptor. In early HIV infection, CD8+ T cell levels may actually rise as the body attempts to kill virus-infected cells. * **B cells:** HIV does not directly infect B cells. However, B cell dysfunction occurs secondary to the loss of T-helper cell signals, leading to paradoxical hypergammaglobulinemia but a poor antibody response to new antigens. **High-Yield Clinical Pearls for NEET-PG:** * **Normal CD4:CD8 ratio** is 2:1. In AIDS, this ratio is **inverted** (less than 1:1). * **Diagnosis of AIDS:** Defined when the CD4 count falls below **200 cells/mm³** or the presence of an AIDS-defining illness. * **Macrophages** act as the "reservoir" for HIV in the body because they are infected but not rapidly destroyed. * **CCR5 Delta 32 mutation:** Individuals with this homozygous mutation are resistant to HIV infection.

Question 76: Full-blown immunodeficiency syndrome is characterized by which of the following?

A. High viral titres with low CD4 count (Correct Answer)
B. Low viral titres with low CD4 count
C. High viral titres with high CD4 count
D. High CD4 and high CD8 count

Explanation: **Explanation:** The progression of HIV infection to **Full-blown AIDS (Acquired Immunodeficiency Syndrome)** is defined by the critical failure of the immune system and uncontrolled viral replication. **Why Option A is Correct:** The hallmark of clinical AIDS is a **CD4+ T-lymphocyte count of <200 cells/mm³** (or a CD4 percentage <14%). As the immune system collapses, it loses the ability to suppress HIV replication. This leads to a **high viral load (titre)** in the plasma. The inverse relationship between CD4 count and viral load is the defining characteristic of the terminal stage of the disease, making the patient highly susceptible to opportunistic infections and malignancies. **Why Other Options are Incorrect:** * **Option B:** Low viral titres with low CD4 counts are typically seen in patients on effective **Highly Active Antiretroviral Therapy (HAART)**, where the virus is suppressed even if the immune system hasn't fully recovered. * **Option C:** High viral titres with high CD4 counts occur during the **Acute Retroviral Syndrome** (initial infection phase), where there is a spike in viremia before the immune system mounts a response. * **Option D:** High CD4 and CD8 counts represent a healthy or robust immune response, the opposite of immunodeficiency. **High-Yield Clinical Pearls for NEET-PG:** * **Indicator of Prognosis:** Plasma HIV RNA levels (Viral Load) are the best predictor of disease progression. * **Indicator of Immune Status:** CD4+ T-cell count is the best indicator of immediate risk for opportunistic infections. * **Inversion of Ratio:** In AIDS, the normal **CD4:CD8 ratio (typically 2:1)** is reversed (becomes <1:1). * **Commonest Opportunistic Infection:** Globally, Tuberculosis is the most common; however, *Pneumocystis jirovecii* pneumonia (PCP) is a classic AIDS-defining illness when CD4 falls below 200.

Question 77: A 7-month-old baby who is failing to thrive is brought into a clinic. The baby's mother died of AIDS 2 months ago. Blood is obtained and sent to the laboratory to check for HIV infection. The physician orders a test whose detection system is based on enzymatic activity. Which of the following tests is a heterogeneous immunoassay?

A. Coagglutination (COA)
B. Counter immuno-electrophoresis (CIE)
C. Enzyme-linked immunosorbent assay (ELISA) (Correct Answer)
D. Latex agglutination (LA)

Explanation: ### Explanation **Correct Answer: C. Enzyme-linked immunosorbent assay (ELISA)** **Why ELISA is the correct answer:** The question describes a clinical scenario (suspected pediatric HIV) and asks for a test based on **enzymatic activity** that is also a **heterogeneous immunoassay**. * **Heterogeneous Immunoassays** require a physical separation step (usually washing) to remove unbound antigens or antibodies from the bound complexes before measuring the signal. * **ELISA** utilizes an enzyme-labeled antibody or antigen. After the binding phase, a washing step is performed (making it heterogeneous) to remove unbound components. A substrate is then added, which the enzyme converts into a colored product, allowing for quantification. **Analysis of Incorrect Options:** * **A. Coagglutination (COA):** This is an agglutination reaction using *Staphylococcus aureus* (Cowan 1 strain) which possesses Protein A. Protein A binds to the Fc portion of IgG, leaving the Fab sites free to react with specific antigens. It does not involve enzymatic activity. * **B. Counter immuno-electrophoresis (CIE):** This is a modification of the precipitation reaction in agar where an electric current is used to drive the antigen and antibody toward each other. It is based on electrophoresis, not enzymes. * **D. Latex agglutination (LA):** This involves coating antigen or antibody onto the surface of latex particles. Visible clumping (agglutination) occurs upon reaction. It is a rapid, non-enzymatic test. **Clinical Pearls for NEET-PG:** * **HIV Diagnosis in Infants:** In a 7-month-old, maternal IgG antibodies can persist, leading to false positives on ELISA. Therefore, **HIV DNA PCR** is the gold standard for diagnosis in infants <18 months. * **Homogeneous vs. Heterogeneous:** Homogeneous assays (e.g., EMIT) do not require a washing step, whereas heterogeneous assays (e.g., ELISA, RIA) do. * **ELISA Generations:** 4th generation ELISA tests detect both **p24 antigen** and **HIV antibodies**, significantly shortening the window period.

Question 78: Which of the following is an example of a CXC chemokine?

A. IL-8 (Correct Answer)
B. IL-2
C. Lymphotactin
D. Fractalkine

Explanation: Chemokines are a family of small cytokines classified into four main groups based on the arrangement of conserved cysteine (C) residues at their amino-terminus. **Correct Option: A (IL-8)** IL-8 (now known as **CXCL8**) is the prototypical **CXC (alpha) chemokine**. In this group, the first two cysteine residues are separated by a single intervening amino acid (X). IL-8 is primarily produced by macrophages and endothelial cells; its primary function is the **potent chemoattraction and activation of neutrophils**. **Incorrect Options:** * **B (IL-2):** This is a classic interleukin (cytokine) involved in T-cell proliferation, not a chemokine. It does not possess the structural cysteine motifs characteristic of the chemokine family. * **C (Lymphotactin):** This belongs to the **C (gamma) chemokine** group (XCL1). These lack the first and third of the four conserved cysteines. It specifically targets T-cell precursors. * **D (Fractalkine):** This is the only member of the **CX3C (delta) chemokine** group (CX3CL1). It has three intervening amino acids between the first two cysteines and exists in both membrane-bound and soluble forms, aiding in cell adhesion. **High-Yield NEET-PG Pearls:** 1. **CC Chemokines (Beta):** The cysteines are adjacent (e.g., **MCP-1, RANTES, MIP-1α**). They primarily attract monocytes and lymphocytes, but *not* neutrophils. 2. **Mnemonic for IL-8:** "Clean up on **Aisle 8**"—Neutrophils are the "janitors" recruited by IL-8 to clean up acute inflammation. 3. **Receptor Association:** Chemokines act through G-protein-coupled receptors (GPCRs). CXCR4 and CCR5 are critical co-receptors for **HIV entry** into T-cells and macrophages, respectively.

Question 79: Which of the following best describes Natural Killer (NK) cells?

A. Activated macrophages
B. Antibody-activated T cells
C. Cells activated by complement
D. Cells that are independent of antibody activation (Correct Answer)

Explanation: **Explanation:** Natural Killer (NK) cells are a subset of large granular lymphocytes that play a critical role in the innate immune system. Unlike T or B cells, they do not possess antigen-specific receptors (TCR/BCR). Their primary function is to provide a rapid response against virally infected cells and tumor cells without prior sensitization. **Why Option D is Correct:** NK cells are **independent of antibody activation** for their primary killing mechanism. They function based on a "balance of signals" from **activating receptors** and **inhibitory receptors** (which recognize MHC Class I molecules). When a cell lacks MHC Class I (a common occurrence in viral infections or malignancy), the inhibitory signal is lost, and the NK cell undergoes spontaneous degranulation to kill the target cell. **Analysis of Incorrect Options:** * **Option A:** Activated macrophages are phagocytic cells derived from monocytes, not lymphocytes. While NK cells secrete IFN-γ to activate macrophages, they are distinct cell types. * **Option B:** T cells require MHC-restricted antigen presentation for activation. NK cells are "null cells" (CD3 negative) and do not require T-cell activation pathways. * **Option C:** While the complement system can enhance opsonization, NK cell activation is primarily regulated by Killer Immunoglobulin-like Receptors (KIRs) and C-type lectin receptors, not the complement cascade. **NEET-PG High-Yield Pearls:** * **Markers:** NK cells are identified by the presence of **CD56** and **CD16**, and the absence of **CD3**. * **ADCC:** While they can act independently, NK cells can also perform **Antibody-Dependent Cellular Cytotoxicity (ADCC)** via their CD16 receptor (FcγRIII) which binds to IgG-coated cells. * **Cytokine Production:** They are a major source of **IFN-gamma**, which bridges innate and adaptive immunity. * **MHC Class I:** NK cells follow the "Missing Self" hypothesis; they kill cells that fail to express MHC Class I.

Question 80: All IgG subclasses are responsible for coagglutination by binding to S. Aureus protein A, except?

A. IgG1
B. IgG2
C. IgG3 (Correct Answer)
D. IgG4

Explanation: **Explanation:** The core concept behind this question is the interaction between **Staphylococcal Protein A (SpA)** and the **Fc region** of human Immunoglobulins. Protein A is a surface component of *Staphylococcus aureus* that acts as a virulence factor by binding to the Fc portion of IgG molecules, effectively orienting the antibody "upside down." This prevents opsonization and phagocytosis by immune cells. **Why IgG3 is the correct answer:** While Protein A has a high affinity for most IgG subclasses, it **does not bind to IgG3**. This is due to a structural difference in the CH2 and CH3 domains of the IgG3 heavy chain. Specifically, the presence of an **arginine** residue at position 435 in IgG3 (instead of histidine found in IgG1, 2, and 4) creates steric hindrance that prevents binding to Protein A. Therefore, IgG3 cannot participate in the coagglutination reaction used in diagnostic laboratory tests. **Analysis of incorrect options:** * **IgG1, IgG2, and IgG4:** These subclasses all possess the specific amino acid sequence in their Fc region required for high-affinity binding to Protein A. Consequently, they all facilitate coagglutination. **High-Yield NEET-PG Pearls:** * **Coagglutination Test:** Utilizes *S. aureus* (Cowan 1 strain) rich in Protein A to detect specific antigens. * **Placental Transfer:** All IgG subclasses cross the placenta, but **IgG1** is transferred most efficiently. * **Complement Activation:** **IgG3** is the most potent activator of the classical complement pathway, followed by IgG1 and IgG2. **IgG4** does not activate complement. * **Abundance:** IgG1 is the most abundant subclass in serum; IgG4 is the least.

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Immunology — MCQs

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