Immunology Practice Questions

Q: In the thymus, which gene is responsible for the recognition of self-antigens?

AIRE. ### Explanation **Correct Answer: B. AIRE** **Mechanism and Concept:** The **AIRE (Autoimmune Regulator)** gene is a transcription factor expressed primarily in the **medullary thymic epithelial cells (mTECs)**. Its primary role is to promote the expression of "tissue-specific antigens" (TSAs)—proteins normally found only in peripheral organs like the pancreas (insulin) or thyroid (thyroglobulin)—within the thymus. This allows maturing T-cells to be "screened" against self-antigens. If a developing T-cell recognizes these self-antigens with high affinity, it undergoes apoptosis. This process is known as **Negative Selection**, which is crucial for establishing **Central Tolerance**. **Analysis of Incorrect Options:** * **A. NOTCH 1:** This is a cell-surface receptor essential for the **commitment** of hematopoietic stem cells to the T-cell lineage rather than the B-cell lineage. It is involved in early T-cell development, not self-antigen recognition. * **C. Rb (Retinoblastoma gene):** A classic tumor suppressor gene that regulates the cell cycle (G1 to S phase transition). Mutations are associated with retinoblastoma and osteosarcoma. * **D. PTEN:** A tumor suppressor gene that acts as a phosphatase to inhibit the PI3K/AKT/mTOR pathway. It is frequently mutated in various cancers (e.g., prostate, endometrial) but does not regulate thymic tolerance. **Clinical Pearls for NEET-PG:** * **APS-1 (Autoimmune Polyglandular Syndrome Type 1):** Caused by a mutation in the **AIRE gene**. It is characterized by the triad of Chronic Mucocutaneous Candidiasis, Hypoparathyroidism, and Addison’s disease. * **Central Tolerance** occurs in the Thymus (T-cells) and Bone Marrow (B-cells). * **Positive Selection** (MHC restriction) occurs in the thymic **cortex**, while **Negative Selection** (Self-tolerance) occurs primarily in the thymic **medulla**.

Q: Which factor is present in the final common terminal complement pathway?

C5. ### Explanation The complement system consists of three activation pathways (Classical, Alternative, and Lectin) that converge at a single point to form the **Membrane Attack Complex (MAC)**, also known as the terminal pathway. **1. Why C5 is the Correct Answer:** The **terminal pathway** begins with the cleavage of **C5** by C5-convertase into C5a and C5b. C5b then serves as the anchor for the assembly of the MAC (C5b-C9). Since C5 is the first component of this final common sequence, it is the correct factor among the choices representing the terminal pathway. **2. Analysis of Incorrect Options:** * **Option A (C4):** This is a component of the **Classical and Lectin pathways**. It is involved in the formation of C3-convertase (C4b2a) and is not part of the terminal pathway. * **Option B (C3):** This is the **most abundant** complement protein and the point where all three pathways converge *before* the terminal sequence begins. While central, it is considered part of the activation phase, not the terminal MAC assembly. * **Option D (Protein B/Factor B):** This is a unique component of the **Alternative pathway**. It combines with C3b to form the alternative C3-convertase (C3bBb). **3. NEET-PG High-Yield Pearls:** * **MAC Components:** C5b, C6, C7, C8, and C9 (C5b-9). * **Deficiency Fact:** Patients with deficiencies in terminal components (C5-C9) have a high susceptibility to disseminated **Neisseria** infections (Meningitis and Gonorrhea). * **C3 Deficiency:** This is the most severe complement deficiency because it is the central convergence point, leading to recurrent pyogenic infections. * **Anaphylatoxins:** C3a, C4a, and **C5a** (C5a is the most potent and acts as a powerful chemoattractant for neutrophils).

Q: Pantone valentine toxin produced by Staphylococcus is an example for which of the following?

Super antigen. **Explanation:** **Panton-Valentine Leukocidin (PVL)** is a potent exotoxin produced by certain strains of *Staphylococcus aureus*. It belongs to the category of **Superantigens**. 1. **Why it is a Superantigen:** Unlike conventional antigens that are processed and presented in the MHC-II groove to specific T-cells (stimulating <0.01% of T-cells), superantigens bind **directly and non-specifically** to the outer surface of the MHC-II molecule and the Vβ region of the T-cell receptor (TCR). This results in a massive, polyclonal activation of up to 20% of the body's T-cells, leading to a "cytokine storm" (massive release of IL-1, IL-2, TNF-α, and IFN-γ). 2. **Analysis of Incorrect Options:** * **Simple Antigen:** These require processing by Antigen Presenting Cells (APCs) and stimulate a specific, limited immune response. * **Hapten:** These are small, non-immunogenic molecules that only elicit an immune response when attached to a large carrier protein. * **Immunoglobulin:** These are antibodies produced by B-cells to neutralize antigens, not toxins produced by bacteria. **High-Yield Clinical Pearls for NEET-PG:** * **PVL Association:** It is a hallmark of **Community-Acquired MRSA (CA-MRSA)**. * **Clinical Presentation:** PVL causes leukocyte destruction and tissue necrosis. It is strongly associated with **severe necrotizing pneumonia** and recurrent **furunculosis** (skin abscesses). * **Other Staphylococcal Superantigens:** Toxic Shock Syndrome Toxin-1 (TSST-1) and Staphylococcal Enterotoxins (causing food poisoning) are also classic examples. * **Mechanism:** PVL is a pore-forming toxin that specifically targets polymorphonuclear leukocytes (neutrophils).

Q: How are T cell functions assessed?

Migration inhibition test. ### Explanation **Correct Answer: C. Migration Inhibition Test** The **Migration Inhibition Test (MIT)** is a classic functional assay for **Cell-Mediated Immunity (CMI)**. When T cells (specifically Th1 cells) are exposed to an antigen they have been previously sensitized to, they release lymphokines, most notably **Macrophage Migration Inhibition Factor (MIF)**. In an *in vitro* capillary tube system, MIF prevents the normal migration of macrophages. Therefore, a positive test (inhibition of migration) indicates functional T cell activity and delayed-type hypersensitivity (DTH). **Analysis of Incorrect Options:** * **A. Phagocyte Index:** This measures the engulfment capacity of polymorphonuclear leukocytes (neutrophils) and macrophages. It assesses the **innate immune system**, not T cell function. * **B. T cell Count:** This is a **quantitative** measure (usually done via flow cytometry for CD3/CD4/CD8 markers). While it tells you how many T cells are present, it does not assess whether they are functionally active. * **D. Immunoglobulin Index:** This measures antibody levels (IgG, IgA, IgM) produced by B cells/plasma cells. It is an assessment of **Humoral Immunity**. **High-Yield Clinical Pearls for NEET-PG:** * **In vivo assessment of T cells:** The **Skin Test (DTH)** using antigens like Tuberculin (Mantoux), Candidin, or Streptokinase is the most common bedside method. * **Other In vitro T cell functional tests:** Lymphocyte Transformation Test (LTT) and Cytotoxicity assays. * **MIF Source:** It is one of the earliest lymphokines discovered, produced by T-lymphocytes to "lock" macrophages at the site of inflammation. * **Enumeration:** T cells are identified by **CD3** (pan-T cell marker) and were historically identified by the **E-rosette technique** (binding to sheep RBCs).

Q: Which of the following immunoglobulins best fixes complement?

IgM. **Explanation:** The ability of an immunoglobulin to fix complement via the **Classical Pathway** depends on its structure and the availability of binding sites for the **C1q** molecule. [1] **Why IgM is the correct answer:** IgM is the most potent activator of the complement system. It exists primarily as a **pentamer**, meaning it has five basic antibody units joined by a J-chain. For C1q to initiate the complement cascade, it must bind to at least two Fc portions of an antibody simultaneously. Because of its pentameric structure, a single molecule of IgM provides multiple closely spaced Fc regions, allowing it to fix complement with high efficiency. It is often said that "it takes only one molecule of IgM, but thousands of molecules of IgG, to activate complement." **Analysis of incorrect options:** * **IgG:** While IgG (specifically subclasses IgG3, IgG1, and IgG2) can fix complement [1], it exists as a **monomer**. To activate C1q, two or more IgG molecules must settle very close to each other on the antigen surface. This makes it significantly less efficient than IgM. [1] * **IgA:** Primarily involved in mucosal immunity (secretory IgA), it does not activate the classical pathway. It can weakly activate the alternative pathway but is not a primary complement fixer. * **IgD:** Found mainly on the surface of B-cells as a receptor [1]; it does not fix complement. **NEET-PG High-Yield Pearls:** * **Order of Complement Fixation Efficiency:** IgM > IgG3 > IgG1 > IgG2. (Note: IgG4 does **not** fix complement). * **Structure:** IgM is a pentamer (secreted) or monomer (B-cell receptor). [1] * **Clinical Correlation:** IgM is the first antibody produced in a primary immune response and is the most effective at agglutination and complement fixation.

Q: Which interleukin is needed for the differentiation of eosinophils?

IL5. **Explanation:** The correct answer is **IL-5**. Interleukin-5 is a key cytokine produced primarily by Th2 cells and innate lymphoid cells (ILC2). It acts as the primary lineage-specific cytokine for **eosinophils**, regulating their production, differentiation, maturation, and activation in the bone marrow. **Why the other options are incorrect:** * **IL-1:** Produced mainly by macrophages, it is a pro-inflammatory cytokine responsible for inducing fever (endogenous pyrogen) and activating acute-phase reactants. * **IL-2:** Known as the T-cell growth factor, it is produced by Th1 cells and is essential for the proliferation and clonal expansion of T-lymphocytes and NK cells. * **IL-4:** This cytokine induces the differentiation of naive T cells into Th2 cells and promotes **B-cell class switching to IgE** and IgG4. While IL-4 and IL-5 often work together in allergic responses, IL-4 focuses on IgE, while IL-5 focuses on eosinophils. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Interleukins (1-5):** "Hot T-bone stEAk" * **IL-1:** **Hot** (Fever) * **IL-2:** **T**-cell stimulator * **IL-3:** **Bone** marrow stimulator (stimulates all myeloid stem cells) * **IL-4:** Ig**E** production * **IL-5:** Ig**A** production and **E**osinophil activation. * **Clinical Correlation:** Monoclonal antibodies targeting IL-5 (e.g., **Mepolizumab, Reslizumab**) or its receptor (**Benralizumab**) are used in the treatment of severe eosinophilic asthma. * **Eosinophilia:** Commonly seen in "NAACP": Neoplasia, Asthma, Allergy, Collagen vascular diseases, and Parasites (especially helminths).

Question 1

Individuals with hypogammaglobulinemia are at particular risk of infectious diarrhea due to which of the following pathogens?

Accepted Answer

Clostridioides difficile and giardiasis

Answer

Giardiasis and shigellosis

Answer

Clostridioides difficile and Vibrio parahaemolyticus

Answer

Vibrio parahaemolyticus and shigellosis

Question 2

In the thymus, which gene is responsible for the recognition of self-antigens?

Accepted Answer

AIRE

Answer

NOTCH 1

Answer

Rb

Answer

PTEN

Question 3

Which factor is present in the final common terminal complement pathway?

Accepted Answer

C5

Answer

C4

Answer

C3

Answer

Protein B

Question 4

Pantone valentine toxin produced by Staphylococcus is an example for which of the following?

Accepted Answer

Super antigen

Answer

Simple antigen

Answer

Hapten

Answer

Immunoglobulin

Question 5

How are T cell functions assessed?

Accepted Answer

Migration inhibition test

Answer

Phagocyte index

Answer

T cell count

Answer

Immunoglobulin index

Question 6

A super-antigen is a bacterial product that

Accepted Answer

Binds to the beta chain of TCR and MHC class II molecules of APC stimulating T cell activation

Answer

Binds to B7 and CD28 co-stimulatory molecules

Answer

Binds to the CD4 molecule causing T cell activation

Answer

Is presented by macrophages to a larger-than-normal number of T helper CD4 lymphocytes

Question 7

An immunologist is conducting an experiment to understand precipitation reactions. He notes that on adding excessive amount of antibody to a serum sample containing small amount of antigen, the amount of precipitate formed is less, compared to adding a smaller amount of antibody to the same sample. What is this phenomenon called?

Accepted Answer

Pro-zone phenomenon

Answer

Zone of equivalence

Answer

Post-zone phenomenon

Answer

Flocculation phenomenon

Question 8

Which statement is not true about innate immunity?

Accepted Answer

It is influenced by hormones.

Answer

It is dependent on genetic constitution.

Answer

Identical twins have the same degree of resistance.

Answer

It is influenced by exposure to antigen.

Question 9

Which of the following immunoglobulins best fixes complement?

Accepted Answer

IgM

Answer

IgG

Answer

IgA

Answer

IgD

Question 10

Which interleukin is needed for the differentiation of eosinophils?

Accepted Answer

IL5

Answer

IL1

Answer

IL2

Answer

IL4

Immunology — MCQs

Immunology — MCQs

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