Immunology Practice Questions

Q: Large granular lymphocytes are a subset of which cell type?

Lymphocytes. ### Explanation **Correct Answer: D. Lymphocytes** **Concept Overview:** Large Granular Lymphocytes (LGLs) represent a morphologically distinct subset of lymphocytes characterized by a larger size than resting T or B cells, an eccentric nucleus, and the presence of prominent azurophilic granules in the cytoplasm. These granules contain cytotoxic proteins like **perforins and granzymes**. The term LGL primarily refers to two functional groups: 1. **Natural Killer (NK) Cells:** These make up the majority (~85%) of LGLs. They are part of the innate immune system and do not require prior sensitization to kill virus-infected or tumor cells. 2. **Activated CD8+ T-cells (Cytotoxic T-lymphocytes):** A smaller subset of LGLs that are part of the adaptive immune system. --- **Why Incorrect Options are Wrong:** * **A. Neutrophils:** These are granulocytes belonging to the myeloid lineage. While they contain granules, they possess multi-lobed nuclei (polymorphonuclear) and are distinct from the lymphoid lineage. * **B. Macrophages:** These are large mononuclear phagocytes derived from monocytes. They have a "frothy" or vacuolated cytoplasm rather than the specific azurophilic granulation seen in LGLs. * **C. Eosinophils:** These are myeloid cells characterized by coarse, brick-red (eosinophilic) granules and typically a bilobed nucleus. --- **NEET-PG High-Yield Pearls:** * **NK Cell Markers:** CD16 (FcγRIII) and CD56 are the characteristic surface markers for NK cells. * **Mechanism of Action:** NK cells utilize the **"Missing Self" hypothesis**, where they kill cells that lack or have downregulated MHC Class I molecules (a common viral/tumor escape mechanism). * **Clinical Correlation:** **LGL Leukemia** is a rare lymphoproliferative disorder characterized by a persistent increase in these cells, often associated with rheumatoid arthritis and neutropenia (similar to Felty’s syndrome).

Q: C3 convertase acts on which of the following substrates?

C3. ### Explanation **Correct Answer: D. C3** **Concept:** The complement system is a cascade of proteins that mediate inflammation and pathogen clearance. The central event in all three pathways (Classical, Alternative, and Lectin) is the formation of **C3 convertase**. As the name implies, a "convertase" is an enzyme that acts upon its specific substrate. C3 convertase cleaves **C3** into C3a (anaphylatoxin) and C3b (opsonin). This step is the most critical amplification point in the complement cascade. **Analysis of Options:** * **Option A (C4b2b):** This is the **Classical/Lectin pathway C3 convertase** itself. It is the enzyme, not the substrate. It acts *on* C3. * **Option B (C4b2b3b):** This is the **Classical pathway C5 convertase**. It is formed when C3b attaches to the C3 convertase. Its substrate is C5, not C3. * **Option C (C4b):** This is a fragment of C4 that acts as a structural component/scaffold for the formation of the C3 convertase (C4b2b), but it is not the substrate being cleaved by the convertase enzyme. **High-Yield Clinical Pearls for NEET-PG:** * **Alternative Pathway C3 Convertase:** Unlike the classical pathway (C4b2b), the alternative pathway C3 convertase is **C3bBb**. * **C3 Deficiency:** This is the most severe complement deficiency because it compromises all three pathways, leading to recurrent pyogenic infections (e.g., *S. pneumoniae*, *H. influenzae*). * **Anaphylatoxins:** C3a, C4a, and C5a (in increasing order of potency: C5a > C3a > C4a) trigger mast cell degranulation. * **Opsonization:** C3b is the primary opsonin that enhances phagocytosis by binding to CR1 receptors on macrophages.

Q: What is the half-life of IgG?

23 days. **Explanation:** The correct answer is **23 days (Option D)**. **Understanding the Concept:** Immunoglobulin G (IgG) is the most abundant antibody in the serum, accounting for approximately 75-80% of the total pool. It has the **longest half-life** of all immunoglobulins, averaging **23 days** (specifically for subclasses IgG1, IgG2, and IgG4; IgG3 is an exception with a half-life of about 7 days). This prolonged half-life is due to the **neonatal Fc receptor (FcRn)**. This receptor binds to IgG in endosomes, protecting it from lysosomal degradation and recycling it back into the circulation. **Analysis of Incorrect Options:** * **A (5 days):** This is the approximate half-life of **IgA**. * **B (6 days):** This is the approximate half-life of **IgM**. * **C (8 days):** This is the approximate half-life of **IgD** (approx. 3 days) or **IgG3** (approx. 7-9 days), but it does not represent the majority of IgG. **High-Yield Clinical Pearls for NEET-PG:** * **Placental Transfer:** IgG is the **only** immunoglobulin that crosses the placenta, providing passive immunity to the newborn. * **Secondary Response:** IgG is the predominant antibody produced during the **anamnestic (secondary) immune response**. * **Subclasses:** IgG has four subclasses. **IgG1** is the most abundant; **IgG2** is vital for defense against capsulated bacteria; **IgG3** is the most effective complement fixer. * **Agglutination:** IgG is a "late" antibody and is considered a **monomer** (valence of 2). Unlike IgM, it is a poor agglutinator but an excellent opsonin.

Q: Which one of the following microorganisms uses antigenic variation as a major means of evading host defenses?

Borrelia recurrentis. **Explanation:** **Correct Answer: B. Borrelia recurrentis** *Borrelia recurrentis*, the causative agent of louse-borne relapsing fever, is the classic example of a pathogen that utilizes **programmed antigenic variation** to evade the host immune system. The bacteria possess a large repertoire of genes encoding **Variable Large Proteins (VLP)** on their outer membrane. Through gene conversion and rearrangement, the bacteria periodically switch these surface proteins. * **Mechanism:** When the host produces antibodies against the dominant serotype, the bacteria are cleared from the blood (afebrile period). However, a small subpopulation switches to a new antigenic variant that the existing antibodies cannot recognize. These multiply, leading to a new wave of bacteremia and fever (the "relapse"). **Why other options are incorrect:** * **A. Streptococcus pneumoniae:** Evades the immune system primarily through its **polysaccharide capsule**, which prevents phagocytosis. While there are many serotypes, an individual strain does not switch its antigens during a single infection. * **C. Mycobacterium tuberculosis:** Uses **intracellular survival** within macrophages by inhibiting phagosome-lysosome fusion. * **D. Listeria monocytogenes:** Escapes host defenses by using **Listeriolysin O** to break out of the phagosome and moving cell-to-cell via **actin polymerization** (actin rockets). **NEET-PG High-Yield Pearls:** * **Other organisms using antigenic variation:** *Neisseria gonorrhoeae* (pili), *Trypanosoma brucei* (VSG genes), and Influenza virus (antigenic drift/shift). * **Relapsing Fever:** Characterized by 3–10 relapses; diagnosis is made by seeing spirochetes on a **peripheral blood smear** (Giemsa/Wright stain) during the febrile phase. * **Jarisch-Herxheimer reaction:** A common systemic inflammatory response seen shortly after starting antibiotics for *Borrelia* infections.

Q: Which CD marker is characteristically associated with B lymphocytes?

CD 19. ### Explanation **Correct Option: A (CD 19)** CD 19 is considered the most reliable and characteristic marker for **B lymphocytes**. It is expressed on the surface of B cells from the earliest stages of pre-B cell development until the stage just before terminal differentiation into plasma cells. It functions as a co-receptor with CD21 and CD81 to lower the threshold for B-cell antigen receptor signaling. In clinical practice, CD19 is the primary marker used for B-cell quantification in flow cytometry. **Analysis of Incorrect Options:** * **CD 27:** While CD27 is found on B cells, it is specifically a marker for **Memory B cells**. It is not universal to all B cell stages, making CD19 a more characteristic general marker. * **CD 4:** This is a characteristic marker for **Helper T lymphocytes** and is also expressed on monocytes and macrophages. It serves as the primary receptor for HIV entry. * **CD 35:** Also known as Complement Receptor 1 (CR1), it is found on various cells including erythrocytes, neutrophils, and B cells. It is not specific to B lymphocytes. **High-Yield Clinical Pearls for NEET-PG:** * **Pan-B cell markers:** CD19, CD20, and CD22. * **Plasma Cell Marker:** CD138 (Note: Mature plasma cells usually lose CD19 and CD20). * **Pan-T cell marker:** CD3 (associated with the T-cell receptor). * **NK cell markers:** CD16 (FcγRIII) and CD56. * **Rituximab:** A monoclonal antibody used in lymphomas and autoimmune diseases that targets **CD20**. * **Epstein-Barr Virus (EBV) Receptor:** CD21 (CR2) on B cells is the attachment site for EBV.

Q: What type of receptors are present on T cells?

CD4. **Explanation:** The correct answer is **CD4**. T cells are characterized by the presence of specific surface markers known as **Cluster of Differentiation (CD)** molecules. These receptors are essential for antigen recognition and immune signaling. * **Why CD4 is correct:** T cells are broadly divided into two functional subsets based on their surface receptors: **Helper T cells (CD4+)** and **Cytotoxic T cells (CD8+)**. CD4 acts as a co-receptor that binds to the non-polymorphic region of **MHC Class II** molecules on Antigen-Presenting Cells (APCs), facilitating the activation of the T-cell receptor (TCR) complex. **Analysis of Incorrect Options:** * **A & B (IgG and IgD):** These are types of Immunoglobulins (antibodies). While **IgD and IgM** serve as the primary B-cell receptors (BCRs), they are not found on T cells. T cells use a T-cell receptor (TCR) instead of membrane-bound antibodies to recognize antigens. * **D (Prostaglandins):** These are lipid-derived inflammatory mediators, not structural receptors found on the surface of T cells. **High-Yield Clinical Pearls for NEET-PG:** 1. **MHC Restriction:** Remember the **"Rule of 8"**: CD4 cells recognize MHC II (4 × 2 = 8), and CD8 cells recognize MHC I (8 × 1 = 8). 2. **Pan-T cell marker:** **CD3** is the definitive marker present on *all* mature T cells and is associated with the TCR. 3. **HIV Pathogenesis:** The HIV virus specifically targets the **CD4 receptor** (using its gp120 envelope protein) to enter and destroy Helper T cells, leading to immunodeficiency. 4. **Th1 vs Th2:** CD4+ cells further differentiate into Th1 (cell-mediated immunity) and Th2 (humoral immunity) based on the cytokine environment.

Q: Which of the following does not secrete interleukin-1 alpha?

Neutrophil. **Explanation:** Interleukin-1 (IL-1) is a key pro-inflammatory cytokine that exists in two primary forms: **IL-1α** and **IL-1β**. While they share the same receptor, their cellular distribution and release mechanisms differ. **Why Neutrophil is the correct answer:** Neutrophils are primarily producers of **IL-1β**, not IL-1α. IL-1α is typically constitutively expressed in epithelial and mesenchymal cells or produced by specific immune cells like macrophages and lymphocytes. Neutrophils focus on the rapid release of IL-1β via the inflammasome pathway during acute inflammation. Therefore, they are generally considered non-secretors of IL-1α. **Analysis of Incorrect Options:** * **Macrophage:** These are the primary professional sources of both IL-1α and IL-1β. They produce IL-1α as a membrane-bound form and a secreted cytokine to initiate the inflammatory cascade. * **Lymphocyte:** Both B-cells and certain T-cell subsets can produce IL-1α, particularly during activation and interaction with antigen-presenting cells. * **Fibroblast:** These are non-immune cells (mesenchymal) that produce IL-1α, especially in response to local tissue injury, acting as an "alarmin" to signal damage to the immune system. **High-Yield NEET-PG Pearls:** * **IL-1α vs. IL-1β:** IL-1α is often cell-associated (membrane-bound) and acts as an **alarmin** (released upon cell death), whereas IL-1β is strictly secreted and requires cleavage by **Caspase-1** (via the inflammasome). * **Endogenous Pyrogen:** IL-1 is a potent endogenous pyrogen that acts on the hypothalamus to induce fever. * **Clinical Correlation:** **Anakinra** is a recombinant IL-1 receptor antagonist used in the treatment of Rheumatoid Arthritis and Autoinflammatory syndromes.

Q: Which of the following is NOT a pyrogen?

Interleukin-18 (IL-18). **Explanation:** Pyrogens are substances that induce fever by acting on the hypothalamus to increase the thermoregulatory set-point. They are classified into **Exogenous pyrogens** (e.g., LPS/Endotoxin) and **Endogenous pyrogens** (pro-inflammatory cytokines). **Why Interleukin-18 (IL-18) is the correct answer:** While IL-18 belongs to the IL-1 family (similar to IL-1β), its primary role is inducing IFN-γ production and enhancing NK cell activity. Unlike its counterparts, IL-18 does not possess significant pyrogenic properties and is not typically involved in the systemic febrile response. **Analysis of Incorrect Options:** * **Interleukin-1 (IL-1):** Often considered the "master pyrogen," it is the most potent inducer of fever. (Note: IL-18 is the outlier in this family). * **Tumor Necrosis Factor-alpha (TNF-α):** A major endogenous pyrogen that induces fever both directly and by stimulating the release of IL-1. * **Interferon-alpha (IFN-α):** Known to produce "flu-like symptoms," including significant fever, often seen as a side effect during clinical administration for hepatitis or malignancies. * **Interleukin-6 (IL-6):** (Often tested) A key mediator that crosses the blood-brain barrier to stimulate prostaglandin E2 (PGE2) production in the hypothalamus. **High-Yield Clinical Pearls for NEET-PG:** * **The Final Mediator:** PGE2 is the ultimate mediator of fever in the hypothalamus. Aspirin and NSAIDs work by inhibiting Cyclooxygenase (COX), thereby blocking PGE2 synthesis. * **Potency Hierarchy:** IL-1 > TNF-α > IL-6. * **Exogenous Pyrogen:** The most common is the Lipopolysaccharide (LPS) of Gram-negative bacteria (Endotoxin).

Q: Prausnitz-kustner reaction is a

Type I Hypersensitivity. **Explanation:** The **Prausnitz-Küstner (PK) reaction** is a classic historical demonstration of **Type I Hypersensitivity** (Immediate Hypersensitivity). It involves the passive transfer of serum from an allergic individual (containing IgE antibodies, formerly called "reagins") into the skin of a non-allergic recipient. When the specific allergen is later injected into the same site, a wheal-and-flare reaction occurs within minutes, proving that the allergic mediator is present in the serum. **Why the other options are incorrect:** * **Type II (Cytotoxic):** Involves IgG or IgM antibodies binding to antigens on cell surfaces (e.g., ABO incompatibility, Myasthenia Gravis). It does not involve IgE-mediated mast cell degranulation. * **Type III (Immune-complex):** Mediated by antigen-antibody complexes depositing in tissues (e.g., SLE, Arthus reaction). These reactions typically take 3–10 hours to manifest. * **Type IV (Delayed-type):** Cell-mediated immunity involving T-lymphocytes rather than antibodies (e.g., Mantoux test, Contact dermatitis). These reactions peak at 48–72 hours. **High-Yield Clinical Pearls for NEET-PG:** * **Mediator:** The PK reaction specifically identifies **IgE** (the only heat-labile antibody). * **Mechanism:** IgE binds to **FcεRI receptors** on mast cells in the recipient's skin. * **Safety Note:** This test is no longer used clinically due to the risk of transmitting blood-borne pathogens (like HIV or Hepatitis B/C). * **Casoni’s Test:** Another example of a Type I Hypersensitivity skin test used for Hydatid disease.

Question 1

Large granular lymphocytes are a subset of which cell type?

Accepted Answer

Lymphocytes

Answer

Neutrophils

Answer

Macrophages

Answer

Eosinophils

Question 2

C3 convertase acts on which of the following substrates?

Accepted Answer

C3

Answer

C4b2b

Answer

C4b2B3b

Answer

C4b

Question 3

What is the half-life of IgG?

Accepted Answer

23 days

Answer

5 days

Answer

6 days

Answer

8 days

Question 4

Which one of the following microorganisms uses antigenic variation as a major means of evading host defenses?

Accepted Answer

Borrelia recurrentis

Answer

Streptococcus pneumoniae

Answer

Mycobacterium tuberculosis

Answer

Listeria monocytogenes

Question 5

Which portion of an antibody molecule binds to an antigen?

Accepted Answer

Hypervariable region

Answer

Hinge region

Answer

Constant region

Answer

Variable region

Question 6

Which CD marker is characteristically associated with B lymphocytes?

Accepted Answer

CD 19

Answer

CD 27

Answer

CD 4

Answer

CD 35

Question 7

What type of receptors are present on T cells?

Accepted Answer

CD4

Answer

IgG

Answer

IgD

Answer

Prostaglandins

Question 8

Which of the following does not secrete interleukin-1 alpha?

Accepted Answer

Neutrophil

Answer

Lymphocyte

Answer

Fibroblast

Answer

Macrophage

Question 9

Which of the following is NOT a pyrogen?

Accepted Answer

Interleukin-18 (IL-18)

Answer

Interferon-alpha (IFN-α)

Answer

Tumor Necrosis Factor-alpha (TNF-α)

Answer

Interleukin-4 (IL-4)

Question 10

Prausnitz-kustner reaction is a

Accepted Answer

Type I Hypersensitivity

Answer

Type II Hypersensitivity

Answer

Type III Hypersensitivity

Answer

Type IV Hypersensitivity

Immunology — MCQs

Immunology — MCQs

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