Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 311: Plasma cells produce specific antibodies by the process of?

A. Class selection
B. Clonal selection (Correct Answer)
C. Isotope selection
D. Isotope switching

Explanation: **Explanation:** The correct answer is **Clonal Selection**. This fundamental immunological principle, proposed by Frank Macfarlane Burnet, explains how the immune system responds to specific antigens. 1. **Why Clonal Selection is Correct:** When a specific antigen enters the body, it "selects" a pre-existing B-lymphocyte that possesses a matching surface receptor (BCR). Once activated, this specific B-cell undergoes rapid proliferation (cloning) and differentiates into **plasma cells**. These plasma cells then secrete large quantities of antibodies with the exact same specificity as the original receptor to neutralize the pathogen. 2. **Analysis of Incorrect Options:** * **Class Selection:** This is a distractor term and not a recognized physiological process in immunology. * **Isotope Selection:** This is an incorrect term. The correct term is "Isotype." * **Isotype Switching (Class Switching):** This occurs *after* clonal selection. It is the process where a B-cell changes the constant region of the antibody heavy chain (e.g., switching from IgM to IgG) to alter the effector function, but the **antigen specificity remains the same**. It does not explain how specific antibodies are produced initially. **High-Yield Clinical Pearls for NEET-PG:** * **Memory Cells:** Clonal selection also produces memory B-cells, which provide a faster and more robust response upon re-exposure (Secondary Immune Response). * **Plasma Cell Morphology:** Characterized by an eccentric nucleus with a "cartwheel" or "clock-face" appearance and a prominent perinuclear halo (Golgi apparatus). * **Malignancy:** A monoclonal proliferation of a single plasma cell clone results in **Multiple Myeloma**, characterized by the production of M-proteins.

Question 312: Which immunoglobulin is scarce in human serum?

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

Explanation: **Explanation:** The concentration of immunoglobulins in human serum follows a specific hierarchy based on their physiological roles and half-lives. **IgE** is the correct answer because it is the least abundant immunoglobulin in the serum, with a concentration of approximately **0.0003 mg/mL** (less than 0.001% of total serum antibodies). This scarcity is due to its high affinity for FcεRI receptors on mast cells and basophils, where it remains sequestered rather than circulating freely. **Analysis of Options:** * **IgG (Option B):** The most abundant immunoglobulin (75–80%). It is the only antibody that crosses the placenta and provides long-term immunity. * **IgA (Option A):** The second most common (10–15%). It is the primary secretory antibody found in colostrum, saliva, and mucosal surfaces. * **IgM (Option C):** Comprises about 5–10%. It is the largest (pentamer) and the first antibody produced in response to an acute infection. * **IgD:** Though not listed, IgD is also scarce (~0.2%), but IgE remains the least concentrated of all five classes. **NEET-PG High-Yield Pearls:** * **Mnemonic for Serum Concentration:** **G > A > M > D > E** ("GAMDE"). * **IgE Function:** Mediates Type I Hypersensitivity (allergic) reactions and provides immunity against helminthic (parasitic) infections. * **Heat Lability:** IgE is uniquely heat-labile (inactivated at 56°C for 30 minutes). * **Prausnitz-Küstner (PK) Reaction:** A historical test used to detect IgE-mediated hypersensitivity.

Question 313: Which immunoglobulin is involved in anaphylaxis?

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

Explanation: **Explanation:** **Correct Answer: B. IgE** The correct answer is IgE because it is the primary mediator of **Type I Hypersensitivity reactions** (Anaphylaxis). When an allergen enters the body, it stimulates B-cells to produce IgE, which then binds to high-affinity receptors (**FcεRI**) on the surface of **mast cells and basophils**. Upon re-exposure, the allergen cross-links these surface-bound IgE molecules, triggering degranulation and the release of potent inflammatory mediators like histamine, leukotrienes, and prostaglandins, leading to systemic anaphylaxis. **Analysis of Incorrect Options:** * **IgA:** Primarily found in secretions (tears, saliva, colostrum, and GI mucus). It provides **mucosal immunity** and prevents the attachment of pathogens to epithelial surfaces. * **IgG:** The most abundant antibody in serum. It is involved in Type II and Type III hypersensitivity, opsonization, and toxin neutralization. It is the only antibody that **crosses the placenta**. * **IgM:** The first antibody produced in a primary immune response. It is a pentamer and is highly effective at **complement activation** and agglutination. **High-Yield Clinical Pearls for NEET-PG:** * **Prausnitz-Küstner (PK) reaction:** A classic test used to demonstrate IgE-mediated hypersensitivity. * **Heat Lability:** IgE is the most heat-labile immunoglobulin (inactivated at 56°C for 30 minutes). * **Reaginic Antibody:** IgE is also known as the reaginic antibody. * **Parasitic Infections:** IgE levels are characteristically elevated in helminthic infections (e.g., Ascariasis) as it mediates eosinophil-induced destruction of parasites.

Question 314: Which immunoglobulin is present on the surface of B lymphocytes and may function as an antigen receptor?

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

Explanation: **Explanation:** The correct answer is **IgD**. **Why IgD is correct:** IgD is primarily found on the surface of **naive B lymphocytes** (B cells that have not yet encountered an antigen), where it co-expresses with IgM. Its primary physiological role is to serve as an **antigen receptor**, signaling the B cell to activate, differentiate, and initiate the humoral immune response. Unlike other antibodies, IgD is secreted into the serum in only trace amounts and has no known major effector function (like opsonization or complement activation) in its soluble form. **Why the other options are incorrect:** * **IgA:** Primarily found in secretions (tears, saliva, colostrum, GI tract) as a dimer. It provides mucosal immunity but is not a primary surface receptor for naive B cells. * **IgE:** Involved in Type I hypersensitivity (allergic) reactions and defense against helminthic parasites. It binds to high-affinity receptors on mast cells and basophils, not as a primary antigen receptor on B cells. * **IgM:** While monomeric IgM *is* also present on the surface of naive B cells as a receptor, the question specifically highlights the characteristic function of IgD. In many competitive exams, if both are listed, IgD is often the preferred answer for "surface receptor function" due to its almost exclusive role in that capacity. **High-Yield Clinical Pearls for NEET-PG:** * **B-Cell Markers:** Naive B cells express both **mIgM and mIgD**. Loss of IgD expression usually indicates B-cell activation and class switching. * **Structure:** IgD has a long hinge region, making it susceptible to proteolysis. * **Isotype Switching:** This process changes the heavy chain constant region (e.g., from IgM/IgD to IgG, IgA, or IgE) but maintains the same antigen specificity. * **Memory B cells:** Usually lack IgD.

Question 315: The opsonic index is related to which process?

A. Phagocytosis (Correct Answer)
B. Vasodilatation
C. Apoptosis
D. Necrosis

Explanation: **Explanation:** The **Opsonic Index** is a measure used to assess the efficiency of **phagocytosis**. It is defined as the ratio of the phagocytic activity of a patient’s blood (in the presence of their serum) compared to the phagocytic activity of a healthy individual’s blood. **Opsonization** is the process where "opsonins" (such as IgG antibodies and C3b complement components) coat a pathogen, acting as "tags" that make the microorganism more attractive and easier to ingest by phagocytes (neutrophils and macrophages). Therefore, the opsonic index directly reflects the capacity of the immune system to perform phagocytosis. **Analysis of Incorrect Options:** * **B. Vasodilatation:** This is a vascular response seen in acute inflammation, primarily mediated by histamine and prostaglandins, not by opsonins. * **C. Apoptosis:** This refers to programmed cell death, a regulated process of cellular suicide that does not involve the opsonization of external pathogens. * **D. Necrosis:** This is accidental or pathological cell death resulting from irreversible injury, characterized by cell swelling and membrane rupture, unrelated to the opsonic index. **High-Yield Facts for NEET-PG:** * **Key Opsonins:** The most important opsonins are **IgG** (specifically the Fc portion) and **C3b** (the "heat-labile" opsonin). * **Phagocytic Index:** This is the average number of bacteria ingested by a single phagocyte. * **Clinical Significance:** A low opsonic index may be seen in certain immunodeficiency states or severe infections where complement or antibodies are depleted. * **Mnemonic:** "Opsonin" comes from the Greek word *opson* (to prepare food for), meaning it "makes the bacteria tasty" for the phagocyte.

Question 316: An infant is diagnosed with Bruton's agammaglobulinemia. Which of the following pathogens poses the most serious threat to this child?

A. Measles virus
B. Mycobacterium tuberculosis
C. Chlamydia trachomatis (Correct Answer)
D. Varicella-zoster virus (VZV)

Explanation: **Explanation:** **Bruton’s Agammaglobulinemia (X-linked Agammaglobulinemia)** is characterized by a mutation in the **BTK gene**, leading to a failure of B-cell maturation. This results in a near-total absence of B-cells and all classes of antibodies (IgG, IgA, IgM). **Why Chlamydia trachomatis is the correct answer:** The primary defense against extracellular bacteria and certain intracellular pathogens like *Chlamydia* relies heavily on **humoral immunity (antibodies)** for opsonization and neutralization. In the absence of secretory IgA and serum IgG, patients are highly susceptible to infections of the mucous membranes. *Chlamydia trachomatis* is a significant threat because its clearance depends on an intact antibody response to prevent attachment and facilitate phagocytosis. **Why the other options are incorrect:** * **Measles, VZV, and M. tuberculosis:** These pathogens are primarily controlled by **Cell-Mediated Immunity (T-cells)**. In Bruton’s, T-cell function remains intact. Therefore, these children usually handle most viral infections (like Measles and VZV) and mycobacterial infections relatively well, as their cytotoxic T-cells and delayed-type hypersensitivity reactions are functional. **Clinical Pearls for NEET-PG:** * **Classic Presentation:** A male infant (X-linked) presenting after 6 months of age (once maternal IgG wanes) with recurrent pyogenic infections (e.g., *S. pneumoniae, H. influenzae*). * **Physical Exam Sign:** Absent or hypoplastic tonsils and adenoids (due to lack of B-cells). * **Diagnosis:** Flow cytometry showing **absent CD19/CD20+ B-cells** with normal T-cell counts. * **Contraindication:** Live viral vaccines (especially **OPV**) are contraindicated as they can cause paralytic disease in these patients.

Question 317: Giardia lambia infection is common in which of the following conditions?

A. Selective IgM deficiency
B. Selective IgA deficiency (Correct Answer)
C. Common variable hypogammaglobulinemia
D. Severe combined immunodeficiency

Explanation: **Explanation:** The correct answer is **Selective IgA deficiency**. **Why Selective IgA deficiency is correct:** Secretory IgA (sIgA) is the primary immunoglobulin responsible for mucosal immunity. It acts by neutralizing pathogens and preventing their attachment to the epithelial lining of the respiratory and gastrointestinal tracts. *Giardia lamblia* is an intestinal protozoan that colonizes the small intestine. In individuals with Selective IgA deficiency—the most common primary immunodeficiency—the lack of mucosal IgA allows *Giardia* to easily adhere to the intestinal wall, leading to chronic or recurrent giardiasis and malabsorption. **Analysis of Incorrect Options:** * **Selective IgM deficiency:** This is a rare condition. While IgM is the first antibody produced in an immune response, its deficiency is more commonly associated with overwhelming sepsis by encapsulated bacteria (e.g., *S. pneumoniae*) rather than specific protozoal intestinal infections. * **Common Variable Immunodeficiency (CVID):** While CVID patients can also get *Giardia* due to low IgA levels, they have a global deficiency of IgG, IgA, and often IgM. Selective IgA deficiency is the more specific and classic association taught for isolated recurrent giardiasis. * **Severe Combined Immunodeficiency (SCID):** SCID involves a total lack of T-cell and B-cell function. These infants present with life-threatening opportunistic infections (e.g., *Pneumocystis jirovecii*, *Candida*) much more severe than isolated giardiasis. **High-Yield Clinical Pearls for NEET-PG:** * **Most common primary immunodeficiency:** Selective IgA deficiency. * **Clinical Presentation:** Most patients are asymptomatic, but they are prone to **sinopulmonary infections**, **Giardiasis**, and **Celiac disease**. * **Contraindication:** Patients with Selective IgA deficiency are at high risk for **Anaphylaxis during blood transfusions** because they develop anti-IgA antibodies. * **Diagnosis:** Serum IgA levels < 7 mg/dL with normal IgG and IgM levels.

Question 318: Which of the following is a recombinant form of GM-CSF?

A. Filgrastim
B. Sargramostim (Correct Answer)
C. Aldesleukin
D. Interleukin-11

Explanation: ### Explanation The correct answer is **Sargramostim**. **1. Why Sargramostim is correct:** Sargramostim is a recombinant form of **Granulocyte-Macrophage Colony-Stimulating Factor (GM-CSF)**. It is produced using yeast (*Saccharomyces cerevisiae*) expression systems. It works by stimulating the proliferation and differentiation of hematopoietic progenitor cells into neutrophils, monocytes/macrophages, and eosinophils. Clinically, it is used to accelerate myeloid recovery following chemotherapy, bone marrow transplantation, or in cases of mobilization of peripheral blood progenitor cells. **2. Analysis of Incorrect Options:** * **Filgrastim:** This is a recombinant form of **G-CSF** (Granulocyte Colony-Stimulating Factor). Unlike GM-CSF, it specifically stimulates the production of neutrophils only. It is more commonly used in clinical practice for chemotherapy-induced neutropenia. * **Aldesleukin:** This is a recombinant form of **Interleukin-2 (IL-2)**. It promotes the proliferation of T-cells and Natural Killer (NK) cells and is primarily used in the treatment of metastatic renal cell carcinoma and melanoma. * **Interleukin-11 (Oprelvekin):** This is a recombinant cytokine used to stimulate **megakaryocytopoiesis**, leading to increased platelet production. It is used to prevent severe thrombocytopenia following chemotherapy. **3. NEET-PG High-Yield Pearls:** * **Mnemonic:** Sar**gram**ostim = **Gran**ulocyte + **M**acrophage (GM-CSF). **Fil**grastim = Neutro**phil** (G-CSF). * **Side Effects:** Sargramostim can cause a "first-dose reaction" (hypotension, tachycardia, flushing) and pleural/pericardial effusions, which are less common with Filgrastim. * **Source:** Filgrastim is produced in *E. coli*, whereas Sargramostim is produced in yeast.

Question 319: What is the most important inflammatory mediator?

A. TNF (Tumor Necrosis Factor) (Correct Answer)
B. IL-2 (Interleukin-2)
C. Interferon
D. PAF (Platelet-Activating Factor)

Explanation: **Explanation:** **TNF (Tumor Necrosis Factor)**, specifically TNF-alpha, is considered the "master regulator" of the inflammatory response. It is primarily produced by activated macrophages and T-cells. Its central role stems from its ability to stimulate the release of other pro-inflammatory cytokines (like IL-1 and IL-6), induce the expression of adhesion molecules on vascular endothelium (facilitating leukocyte migration), and trigger the acute-phase response. In high concentrations, it is the chief mediator of septic shock and systemic inflammatory response syndrome (SIRS). **Analysis of Incorrect Options:** * **IL-2 (Interleukin-2):** Primarily functions as a T-cell growth factor. It is essential for the proliferation and clonal expansion of T-lymphocytes rather than initiating the general inflammatory cascade. * **Interferons (IFNs):** These are primarily involved in antiviral responses (IFN-alpha/beta) and activating macrophages (IFN-gamma). While important in immunity, they are not the primary drivers of acute inflammation. * **PAF (Platelet-Activating Factor):** A potent phospholipid mediator that causes platelet aggregation and vasodilation. While significant in inflammation and anaphylaxis, its scope and regulatory influence are narrower compared to TNF. **NEET-PG High-Yield Pearls:** * **TNF-alpha** is the most important mediator in **Septic Shock**. * **IL-1** is the primary mediator of **fever** (endogenous pyrogen). * **IL-8** is the most potent **chemotactic agent** for neutrophils. * **Anti-TNF drugs** (e.g., Infliximab, Etanercept) are used clinically to treat chronic inflammatory conditions like Rheumatoid Arthritis and Crohn’s disease.

Question 320: The tuberculin test:

A. Depends on activation of naive CD8 T cells by tuberculin antigen
B. Requires rapid activation of naive CD4 T cells
C. Involves no antigen presentation at the injection site
D. Is dependent on memory Th1 cells (Correct Answer)

Explanation: The Tuberculin Skin Test (Mantoux test) is the classic clinical example of a **Type IV (Delayed-Type) Hypersensitivity reaction**. ### Why the Correct Answer is Right The test relies on **cell-mediated immunity**. When Purified Protein Derivative (PPD) is injected intradermally, it is processed by local antigen-presenting cells (APCs). In an individual previously exposed to *M. tuberculosis*, **effector memory Th1 cells (CD4+)** recognize the antigen. These memory cells secrete cytokines (primarily **IFN-γ** and **TNF-α**), which recruit and activate macrophages, leading to local induration and erythema within 48–72 hours. ### Why Other Options are Wrong * **Option A:** The reaction is primarily mediated by **CD4+ T cells**, not CD8+ T cells. CD8+ cells are more involved in direct cytotoxicity (e.g., viral infections or graft rejection). * **Option B:** The test depends on **memory cells**, not naive cells. Naive cells would take weeks to prime a primary immune response; the 48-72 hour window is characteristic of a secondary response by pre-existing memory cells. * **Option C:** Antigen presentation is **essential**. Local macrophages and dendritic cells (Langerhans cells) must present the PPD via MHC Class II molecules to trigger the memory Th1 cells. ### High-Yield Clinical Pearls for NEET-PG * **Timing:** Maximum response occurs at **48–72 hours** (hence "delayed"). * **False Negatives (Anergy):** Can occur in miliary TB, sarcoidosis, malnutrition, Hodgkin’s lymphoma, and AIDS (due to low CD4 counts). * **False Positives:** Seen in individuals vaccinated with **BCG** or those with atypical mycobacterial infections. * **Key Cytokine:** **IFN-γ** is the most critical cytokine involved in activating macrophages to form granulomas.

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Immunology — MCQs

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