Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 741: What is the common function of neutrophils, monocytes, and macrophages?

A. Immune response
B. Phagocytosis (Correct Answer)
C. Liberation of histamine
D. Destruction of old erythrocytes

Explanation: ### Explanation **1. Why Phagocytosis is the Correct Answer:** Phagocytosis is the primary mechanism of the **innate immune system** used to ingest and destroy particulate matter, such as bacteria, debris, and foreign antigens. * **Neutrophils** (Polymorphonuclear leukocytes) are the "first responders" that migrate to the site of acute inflammation to perform rapid phagocytosis. * **Monocytes** are circulating precursors that migrate into tissues to differentiate into **Macrophages**. * Together, these cells are known as **professional phagocytes**. They utilize receptors (like Fc and C3b receptors) to recognize opsonized pathogens, engulf them into phagosomes, and fuse them with lysosomes containing hydrolytic enzymes and reactive oxygen species (ROS) to achieve microbial killing. **2. Analysis of Incorrect Options:** * **A. Immune response:** While these cells participate in the immune response, this is a broad category rather than a specific *function*. Phagocytosis is the specific physiological process they all share. * **C. Liberation of histamine:** This is the primary function of **Mast cells and Basophils** during Type I hypersensitivity reactions and acute inflammation. * **D. Destruction of old erythrocytes:** This is a specialized function of the **Splenic Macrophages** (part of the Reticuloendothelial System). Neutrophils do not typically participate in the physiological recycling of red blood cells. **3. High-Yield Clinical Pearls for NEET-PG:** * **Opsonization:** The most important opsonins that enhance phagocytosis are **IgG** and **C3b** ("IgG and C3b make bacteria tasty"). * **Respiratory Burst:** The process by which phagocytes increase oxygen consumption to produce superoxide radicals via **NADPH oxidase**. A deficiency in this enzyme leads to **Chronic Granulomatous Disease (CGD)**. * **Chediak-Higashi Syndrome:** A defect in microtubule polymerization that prevents phagosome-lysosome fusion, leading to giant granules in neutrophils and recurrent infections.

Question 742: Which of the following diagnostic methods does not utilize labeled antibodies?

A. Enzyme Linked Immunosorbent Assay (ELISA)
B. Hemagglutination inhibition test (Correct Answer)
C. Radioimmunoassay (RIA)
D. Immunofluorescence

Explanation: ### Explanation The core concept in this question is distinguishing between **labeled** and **unlabeled** antigen-antibody reactions. **1. Why Hemagglutination Inhibition (HI) is the correct answer:** The Hemagglutination Inhibition test is an **unlabeled** serological assay. It relies on the biological property of certain viruses (like Influenza or Rubella) to naturally agglutinate red blood cells (RBCs). In this test, if specific antibodies are present in the patient's serum, they bind to the virus and **prevent (inhibit)** it from agglutinating the RBCs. The "indicator" here is the physical settling of RBCs (forming a button), not a chemical, radioactive, or fluorescent tag. **2. Why the other options are incorrect:** * **ELISA (Option A):** Utilizes antibodies labeled with an **enzyme** (e.g., Horseradish peroxidase). The enzyme reacts with a substrate to produce a color change. * **Radioimmunoassay (Option B):** Uses antibodies or antigens labeled with a **radioisotope** (e.g., Iodine-125) to measure very low concentrations of substances. * **Immunofluorescence (Option D):** Employs antibodies labeled with **fluorescent dyes** (e.g., Fluorescein isothiocyanate - FITC) which glow under a UV microscope. **Clinical Pearls for NEET-PG:** * **Labeled assays** (ELISA, RIA, IF) are generally more sensitive than **unlabeled assays** (Precipitation, Agglutination). * **HI Test Application:** It is the gold standard for detecting antibodies against the **Influenza virus** and is also used for Rubella and Dengue. * **Direct vs. Indirect:** If the label is on the primary antibody, it is "Direct"; if it is on the secondary anti-species antibody, it is "Indirect" (the latter is usually more sensitive). * **Coombs Test:** Remember that the Coombs test is a type of **Agglutination** reaction, not a labeled assay.

Question 743: In the thymus, which gene is responsible for the recognition of self-antigens?

A. NOTCH 1
B. AIRE (Correct Answer)
C. Rb
D. PTEN

Explanation: ### 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**.

Question 744: EAC rosettes are formed by which type of cells?

A. B-cells (Correct Answer)
B. T-cells
C. Dendritic cells
D. Osteoclasts

Explanation: **Explanation:** The formation of **EAC rosettes** is a classic laboratory technique used to identify and enumerate **B-lymphocytes**. **1. Why B-cells are the correct answer:** The acronym **EAC** stands for **E**rythrocyte (Sheep RBCs), **A**ntibody (IgM), and **C**omplement (C3). B-cells possess surface receptors for the **C3b component** of the complement (specifically CR1/CD35 and CR2/CD21). In this assay, sheep RBCs are coated with anti-RBC IgM and complement. When incubated with lymphocytes, the B-cells bind to the C3b on the RBCs, forming a "rosette" (a central B-cell surrounded by a ring of RBCs). **2. Why the other options are incorrect:** * **T-cells:** These form **E-rosettes** (Erythrocyte rosettes) when mixed with sheep RBCs directly. This occurs because T-cells express **CD2** receptors, which bind to LFA-3 on sheep erythrocytes. They do not require antibody or complement for this binding. * **Dendritic cells:** While these are potent antigen-presenting cells, they are not identified or quantified using the EAC rosette technique. * **Osteoclasts:** These are multinucleated bone cells derived from the monocyte-macrophage lineage; they do not participate in lymphocyte rosetting assays. **High-Yield Clinical Pearls for NEET-PG:** * **E-Rosette:** Marker for **T-cells** (binds via CD2). * **EAC-Rosette:** Marker for **B-cells** (binds via C3 receptors). * **CD21:** The specific receptor on B-cells used in EAC rosetting; it is also the receptor for the **Epstein-Barr Virus (EBV)**. * Modern identification of these cells is now primarily done via **Flow Cytometry**, but rosette assays remain a favorite high-yield topic in classical immunology exams.

Question 745: Which immunoglobulin is primarily involved in immediate hypersensitivity (Type I) reactions?

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

Explanation: **Explanation:** **Correct Option: C (IgE)** Type I hypersensitivity (Immediate Hypersensitivity) is mediated by **IgE antibodies**. Upon first exposure to an allergen (sensitization), IgE is produced and 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 pharmacological mediators like **histamine**, leukotrienes, and prostaglandins. This results in clinical manifestations ranging from allergic rhinitis and urticaria to life-threatening anaphylaxis. **Incorrect Options:** * **IgG (Option A):** Primarily involved in Type II (cytotoxic) and Type III (immune-complex) hypersensitivity. It is the most abundant antibody in serum and crosses the placenta. * **IgA (Option B):** The primary secretory immunoglobulin found in colostrum, saliva, and tears. It protects mucosal surfaces but does not mediate Type I reactions. * **IgM (Option C):** The first antibody produced in a primary immune response and the most effective at activating the classical complement pathway. It is involved in Type II hypersensitivity (e.g., ABO incompatibility). **High-Yield NEET-PG Pearls:** * **Prausnitz-Küstner (PK) reaction:** A classic test used to demonstrate IgE-mediated skin reactivity. * **Atopy:** A genetic predisposition to produce excessive IgE in response to common environmental allergens. * **Eosinophils:** Recruited during the late-phase response of Type I hypersensitivity via IL-5. * **Treatment:** Epinephrine is the drug of choice for systemic anaphylaxis as it physiologicaly antagonizes histamine.

Question 746: Which factor is present in the final common terminal complement pathway?

A. C4
B. C3
C. C5 (Correct Answer)
D. Protein B

Explanation: ### 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).

Question 747: How are T cell functions assessed?

A. Phagocyte index
B. T cell count
C. Migration inhibition test (Correct Answer)
D. Immunoglobulin index

Explanation: ### 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).

Question 748: What enzyme is required for the conversion of DNA to RNA?

A. DNA Polymerase
B. DNA Ligase
C. RNA polymerase (Correct Answer)
D. Primase

Explanation: **Explanation:** The process of converting DNA into RNA is known as **Transcription**. This is a fundamental step in the central dogma of molecular biology, where genetic information stored in DNA is transcribed into messenger RNA (mRNA), transfer RNA (tRNA), or ribosomal RNA (rRNA). **Why RNA Polymerase is Correct:** **RNA polymerase** is the primary enzyme responsible for transcription. It binds to a specific DNA sequence called the promoter, unwinds the DNA helix, and catalyzes the synthesis of an RNA strand by adding ribonucleotides complementary to the DNA template (in a 5' to 3' direction). In eukaryotes, there are three main types: RNA Pol I (rRNA), RNA Pol II (mRNA), and RNA Pol III (tRNA). **Why Other Options are Incorrect:** * **DNA Polymerase:** This enzyme is used in **DNA Replication**, not transcription. It synthesizes a new DNA strand from a DNA template. * **DNA Ligase:** Known as the "molecular glue," this enzyme joins DNA fragments together (e.g., Okazaki fragments) by forming phosphodiester bonds. * **Primase:** This is a specialized RNA polymerase that creates a short RNA **primer** to provide a 3'-OH group, which is essential for DNA polymerase to begin DNA replication. **Clinical Pearls for NEET-PG:** * **Rifampicin**, a key anti-tubercular drug, acts by inhibiting the **beta-subunit of bacterial DNA-dependent RNA polymerase**. * **Alpha-amanitin** (from the *Amanita phalloides* mushroom) specifically inhibits **RNA Polymerase II**, leading to severe liver failure. * In prokaryotes, a single RNA polymerase (with a **Sigma factor** for initiation) handles all transcription, whereas eukaryotes require three distinct enzymes.

Question 749: A super-antigen is a bacterial product that

A. Binds to B7 and CD28 co-stimulatory molecules
B. Binds to the beta chain of TCR and MHC class II molecules of APC stimulating T cell activation (Correct Answer)
C. Binds to the CD4 molecule causing T cell activation
D. Is presented by macrophages to a larger-than-normal number of T helper CD4 lymphocytes

Explanation: ### Explanation **Super-antigens** are unique bacterial or viral proteins that bypass the traditional rules of antigen processing. **1. Why Option B is Correct:** Normally, an antigen must be internalized, processed into peptides by an Antigen Presenting Cell (APC), and presented within the **groove** of an MHC Class II molecule to a specific T-cell receptor (TCR). In contrast, super-antigens bind **directly and non-specifically** to the **outer surface** of the **MHC Class II** molecule and the **Vβ (beta) chain** of the TCR. Because they bind outside the peptide-binding groove, they can activate any T-cell possessing a particular Vβ segment, regardless of its antigen specificity. This leads to the massive activation of up to 20% of the body’s T-cells (compared to 0.001% in normal responses). **2. Why Other Options are Incorrect:** * **Option A:** B7 (on APC) and CD28 (on T-cell) are standard co-stimulatory signals. Super-antigens do not bind these; they bridge the MHC-II and TCR directly. * **Option C:** Super-antigens do not bind the CD4 molecule itself; they bind the Vβ region of the TCR. * **Option D:** Super-antigens are **not processed** or "presented" by macrophages in the traditional sense. They bind to the MHC-II externally without being internalized or proteolytically degraded. **3. High-Yield Clinical Pearls for NEET-PG:** * **Cytokine Storm:** The massive T-cell activation leads to an overwhelming release of **IFN-γ, IL-2, and TNF-α**, resulting in shock and multi-organ failure. * **Classic Examples:** * *Staphylococcus aureus*: **TSST-1** (Toxic Shock Syndrome Toxin) and Enterotoxins (Food poisoning). * *Streptococcus pyogenes*: **SpeA and SpeC** (Pyrogenic exotoxins causing Streptococcal TSS). * **Key Distinction:** Unlike conventional antigens, super-antigens do not induce immunological memory.

Question 750: Which of the following is NOT a heterophile agglutination test?

A. Weil-Felix test
B. Widal test (Correct Answer)
C. Paul-Bunnel test
D. Streptococcus MG

Explanation: **Explanation:** **Heterophile agglutination tests** are based on the principle of **heterophile antigens**—cross-reacting antigens shared by unrelated species. In these tests, antibodies produced against a specific pathogen react with antigens found on cells of a completely different species (e.g., bacteria, sheep RBCs). * **Why Widal test is the correct answer:** The Widal test is **not** a heterophile test. It is a **specific serological test** used for Enteric fever (Typhoid). It detects antibodies against the specific *Salmonella typhi* and *S. paratyphi* antigens (O and H antigens) using the bacteria themselves as the reagent. There is no cross-reactivity with unrelated species involved. **Analysis of Incorrect Options:** * **Weil-Felix test:** A classic heterophile test for **Rickettsial infections**. Patient antibodies react with antigens of certain strains of **Proteus** (*OX19, OX2, OXK*), which are unrelated to Rickettsia. * **Paul-Bunnel test:** Used to diagnose **Infectious Mononucleosis** (EBV). It detects heterophile antibodies in the patient's serum that cause agglutination of **sheep erythrocytes**. * **Streptococcus MG:** This test was historically used to diagnose **Primary Atypical Pneumonia** caused by *Mycoplasma pneumoniae*. The antibodies produced react with the Streptococcus MG strain (a non-hemolytic streptococcus). **High-Yield Clinical Pearls for NEET-PG:** * **Cold Agglutinin Test:** Another heterophile test for *Mycoplasma pneumoniae* where antibodies agglutinate human **O-group RBCs** at 4°C. * **Weil-Felix Patterns:** * Epidemic Typhus: OX19 (+) * Scrub Typhus: OXK (+) * Q Fever: Negative for all (No Weil-Felix reaction). * **Monospot Test:** A modern rapid latex agglutination version of the Paul-Bunnel test.

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Immunology — MCQs

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