Immunology — MCQs

A 15-year-old male presents with fever (103°F), severe headache, stiff neck, and a petechial rash. A lumbar puncture reveals Gram-negative diplococci (Neisseria meningitidis). Which of the following tests, combining features of gel diffusion and immunoelectrophoresis, is applicable to negatively charged antigens for confirming the causative agent?

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Allergic rhinitis is which type of hypersensitivity reaction?

Immunology Indian Medical PG Practice Questions and MCQs

Question 691: DNA is detected by:

A. Southern blot (Correct Answer)
B. Northern blot
C. Western blot
D. Eastern blot

Explanation: **Explanation:** The detection of specific biological molecules is a fundamental concept in molecular biology and immunology, often tested in NEET-PG via the "Blotting Techniques" mnemonic. **1. Why Southern Blot is Correct:** The **Southern blot** is the gold standard technique used to detect specific **DNA** sequences in a sample. It involves the enzymatic digestion of DNA into fragments, separation by gel electrophoresis, transfer (blotting) to a membrane, and hybridization with a labeled DNA probe. It was named after its inventor, **Edwin Southern**. **2. Why Other Options are Incorrect:** * **Northern Blot:** This technique is used to detect **RNA** (specifically mRNA) to study gene expression. * **Western Blot:** This is used to detect specific **Proteins**. In clinical practice, it is famously used as a confirmatory test for **HIV** (detecting antibodies against p24, gp41, and gp120/160). * **Eastern Blot:** This is a specialized technique used to detect **post-translational modifications** of proteins (such as carbohydrates or lipids). **3. High-Yield Clinical Pearls for NEET-PG:** To remember these easily, use the **"SNOW DROP"** mnemonic: * **S**outhern = **D**NA * **N**orthern = **R**NA * **O** = **O** (No match) * **W**estern = **P**rotein * **Southwestern Blot:** A hybrid technique used to detect **DNA-binding proteins** (e.g., transcription factors like c-Jun or c-Fos). * **Polymerase Chain Reaction (PCR):** While blotting detects DNA, PCR is used to **amplify** DNA. * **ELISA:** Primarily used for protein/antigen/antibody detection; it is the screening test for HIV, whereas Western Blot is the confirmatory test.

Question 692: MHC class III genes encode which of the following?

A. Complement component C3
B. Tumor necrosis factor (Correct Answer)
C. Interleukin 2
D. Beta 2 microglobulin

Explanation: **Explanation:** The Major Histocompatibility Complex (MHC) is a cluster of genes located on the short arm of **Chromosome 6**. While MHC Class I and II are primarily involved in antigen presentation, **MHC Class III** genes are unique because they do not encode cell-surface receptors. Instead, they encode various secreted proteins involved in the immune and inflammatory response. **Why Option B is correct:** MHC Class III genes encode several critical inflammatory mediators, most notably **Tumor Necrosis Factor (TNF-α and TNF-β)**. Additionally, they encode heat shock proteins (HSP) and specific components of the complement system. **Analysis of Incorrect Options:** * **Option A (C3):** While MHC Class III encodes complement components **C2, C4, and Factor B**, it does **not** encode C3. This is a common trap in NEET-PG; remember that C3 is the most abundant complement protein but is not linked to the MHC locus. * **Option C (Interleukin 2):** IL-2 is a T-cell growth factor encoded on Chromosome 4, not Chromosome 6. * **Option D (Beta 2 microglobulin):** This is the light chain of the MHC Class I molecule. Crucially, while the heavy chain of MHC I is encoded on Chromosome 6, $\beta_2$-microglobulin is encoded on **Chromosome 15**. **High-Yield Clinical Pearls for NEET-PG:** * **MHC Class I:** HLA-A, B, C (Present endogenous antigens to CD8+ T cells). * **MHC Class II:** HLA-DP, DQ, DR (Present exogenous antigens to CD4+ T cells). * **MHC Class III:** Includes Complement (C2, C4), TNF, and Heat Shock Proteins. * **MHC Restriction:** CD8+ cells are MHC I restricted; CD4+ cells are MHC II restricted. * **Non-classical MHC:** HLA-G (expressed on cytotrophoblasts) protects the fetus from maternal NK cell attack.

Question 693: Interleukins and growth factors are considered examples of which of the following?

A. Neurotransmitters
B. Metalloproteinases
C. Phosphoproteinases
D. Cytokines (Correct Answer)

Explanation: **Explanation:** **Why Cytokines is the correct answer:** Cytokines are low-molecular-weight, soluble proteins or glycoproteins secreted by cells of both the innate and adaptive immune systems. They act as chemical messengers that regulate the intensity and duration of the immune response by stimulating or inhibiting the activation, proliferation, and differentiation of various cells. **Interleukins (ILs)** are a subset of cytokines originally thought to act only between leukocytes, while **Growth Factors** (like TGF-β) are cytokines that primarily regulate cell survival and proliferation. They function through autocrine, paracrine, or endocrine pathways. **Why other options are incorrect:** * **Neurotransmitters:** These are chemical messengers (e.g., Acetylcholine, Dopamine) released by neurons at synapses to transmit signals to other neurons, muscle cells, or glands. * **Metalloproteinases (MMPs):** These are calcium-dependent, zinc-containing enzymes that degrade extracellular matrix components (e.g., collagenase). They are involved in tissue remodeling, not primary cell signaling. * **Phosphoproteinases (Phosphatases):** These are enzymes that remove phosphate groups from proteins (dephosphorylation), acting as regulatory switches within intracellular signaling pathways rather than extracellular messengers. **High-Yield Clinical Pearls for NEET-PG:** * **Pleiotropy:** One cytokine having multiple different effects on different cell types (e.g., IL-4 acting on B-cells, T-cells, and mast cells). * **Redundancy:** Multiple cytokines exerting the same effect (e.g., IL-2, IL-4, and IL-5 all trigger B-cell proliferation). * **Pro-inflammatory Cytokines:** IL-1, IL-6, and TNF-α (Key mediators of the acute phase response). * **Anti-inflammatory Cytokines:** IL-10 and TGF-β. * **IL-8:** The primary chemotactic factor for neutrophils ("Clean up on aisle 8").

Question 694: Lack of reaction to our own HLA (human leukocyte antigen) is known as?

A. Autoimmunity
B. Clonal selection
C. Tolerance (Correct Answer)
D. Complement system

Explanation: ### Explanation **Correct Answer: C. Tolerance** **Understanding the Concept:** **Immunological tolerance** is the state of unresponsiveness of the immune system to substances or tissues that have the potential to induce an immune response. Specifically, **self-tolerance** is the body's ability to recognize and not attack its own antigens, such as HLA (Human Leukocyte Antigens). This is achieved through two primary mechanisms: 1. **Central Tolerance:** Occurs during lymphocyte development in the thymus (T-cells) and bone marrow (B-cells) via negative selection (deletion of self-reactive clones). 2. **Peripheral Tolerance:** Occurs in the secondary lymphoid organs through mechanisms like anergy (functional inactivation), suppression by T-regulatory cells, or apoptosis. **Why Incorrect Options are Wrong:** * **A. Autoimmunity:** This is the exact opposite of tolerance. It is a clinical state where the immune system fails to recognize "self" and mounts an attack against the body's own HLA or tissues. * **B. Clonal Selection:** This is the process by which a specific antigen activates and expands only those lymphocytes that have receptors specific for that antigen. It explains how we respond to foreign pathogens, not why we ignore self-antigens. * **C. Complement System:** This is a part of the innate immune system consisting of plasma proteins that enhance (complement) the ability of antibodies and phagocytic cells to clear pathogens. It does not dictate self-recognition. **High-Yield Clinical Pearls for NEET-PG:** * **HLA (MHC) Restriction:** T-cells only recognize antigens when presented on self-MHC molecules. * **AIRE Gene:** The Autoimmune Regulator (AIRE) gene is crucial for central tolerance; mutations lead to **APECED** (Autoimmune Polyendocrinopathy-Candidiasis-Ectodermal Dystrophy). * **Anergy:** Occurs when a T-cell recognizes an antigen without the necessary co-stimulatory signal (e.g., CD28-B7 interaction).

Question 695: The variable region of an antibody molecule is located where?

A. C-terminal
B. N-terminal (Correct Answer)
C. Intermediate region
D. Carbohydrate moiety

Explanation: ### Explanation **Correct Answer: B. N-terminal** **Concept Overview:** An antibody (Immunoglobulin) is a Y-shaped glycoprotein consisting of two heavy (H) chains and two light (L) chains. Each chain has a **Variable (V) region** and a **Constant (C) region**. The variable regions of both the heavy and light chains are located at the **amino-terminal (N-terminal)** ends of the polypeptide chains. This region contains the **Antigen Binding Site (Paratope)**, which is characterized by high sequence variability to allow for the recognition of a vast array of specific antigens. **Analysis of Options:** * **A. C-terminal:** The carboxy-terminal (C-terminal) end constitutes the **Constant region** of the antibody. This region determines the biological properties of the immunoglobulin (e.g., complement fixation, placental transfer, and binding to Fc receptors on immune cells). * **C. Intermediate region:** There is no specific "intermediate region" in standard antibody nomenclature. The area between the Fab and Fc portions is known as the **Hinge region**, which provides flexibility but does not house the primary variable sequence. * **D. Carbohydrate moiety:** Most antibodies are glycosylated, but these sugars are typically attached to the **CH2 domain** (Constant region) of the heavy chain and are involved in structural stability and effector functions, not antigen binding. **High-Yield Clinical Pearls for NEET-PG:** * **Hypervariable Regions (CDRs):** Within the N-terminal variable region, there are three "Complementarity Determining Regions" (CDRs) that make direct contact with the antigen. * **Papain Digestion:** Cleaves the antibody *above* the hinge region into **two Fab fragments** (containing the N-terminal/variable regions) and **one Fc fragment**. * **Pepsin Digestion:** Cleaves *below* the hinge region, resulting in one **F(ab')2 fragment** and degraded Fc fragments. * **Isotype Determination:** The C-terminal (Constant region) of the heavy chain determines the class of the antibody (IgG, IgM, IgA, IgD, IgE).

Question 696: Which of the following laboratory assays can be used to detect the number and types of immune cells in the peripheral blood?

A. Immunofixation electrophoresis
B. Immunoblot
C. Enzyme immunoassay (EIA)
D. Flow cytometry (Correct Answer)

Explanation: **Explanation:** **Flow cytometry** is the gold standard for identifying and quantifying specific cell populations in a fluid suspension. It works by passing cells in a single-file stream through a laser beam. Cells are pre-labeled with fluorescent-tagged monoclonal antibodies that bind to specific surface markers (Clusters of Differentiation or **CD markers**). As each cell passes the laser, it scatters light and emits fluorescence, allowing the machine to measure the cell's size (Forward Scatter), granularity (Side Scatter), and specific type (Fluorescence). This makes it the ideal tool for "immunophenotyping"—calculating the absolute number and percentage of T-cells (CD3), B-cells (CD19/20), and NK cells. **Why the other options are incorrect:** * **Immunofixation electrophoresis:** Used to identify specific types of monoclonal antibodies (M-protein) in conditions like Multiple Myeloma. It detects proteins, not whole cells. * **Immunoblot (Western Blot):** Used to detect specific proteins (antigens or antibodies) in a sample (e.g., confirmatory test for HIV). It involves protein separation by molecular weight. * **Enzyme immunoassay (EIA/ELISA):** Used to detect and quantify soluble antigens or antibodies in serum (e.g., screening for Hepatitis B or HIV). It cannot count or differentiate cell populations. **High-Yield Clinical Pearls for NEET-PG:** * **CD4/CD8 Ratio:** Flow cytometry is the primary method used to monitor the CD4 count in HIV/AIDS patients. * **Leukemia/Lymphoma:** It is essential for the "lineage" determination of malignant cells (e.g., distinguishing AML from ALL). * **PNH (Paroxysmal Nocturnal Hemoglobinuria):** Flow cytometry is the most sensitive test to detect the absence of CD55 and CD59 on RBCs/WBCs.

Question 697: Reactive cold agglutinins suggest which of the following diseases?

A. Chronic infectious mononucleosis
B. Primary syphilis
C. Scarlet fever
D. Primary atypical pneumonia (Correct Answer)

Explanation: **Explanation:** **Primary atypical pneumonia**, most commonly caused by *Mycoplasma pneumoniae*, is classically associated with the production of **cold agglutinins**. These are autoantibodies (typically IgM) directed against the **I-antigen** on the surface of human red blood cells. At low temperatures (0–4°C), these antibodies cause RBCs to clump together (agglutinate). While not specific to *Mycoplasma*, a titer of ≥1:64 is a high-yield diagnostic clue in a patient presenting with an insidious onset of non-productive cough, headache, and "patchy" infiltrates on X-ray. **Analysis of Incorrect Options:** * **Chronic infectious mononucleosis:** While EBV can cause cold agglutinins (specifically anti-i antibodies), it is more famously associated with **Heterophile antibodies** (detected by the Paul-Bunnell/Monospot test). * **Primary syphilis:** This is caused by *Treponema pallidum* and is diagnosed via dark-field microscopy or serology (VDRL/RPR). It does not trigger cold agglutinin production. * **Scarlet fever:** Caused by *Streptococcus pyogenes* (Group A Strep) erythrogenic toxin; it presents with a "sandpaper" rash and "strawberry tongue." Diagnosis is via ASLO titers or throat culture. **High-Yield Clinical Pearls for NEET-PG:** * **Target Antigen:** Cold agglutinins in *Mycoplasma* are **Anti-I**, whereas in Infectious Mononucleosis, they are **Anti-i**. * **Complication:** These antibodies can lead to **Autoimmune Hemolytic Anemia (AIHA)** of the "cold" variety. * **Treatment:** *Mycoplasma* lacks a cell wall; therefore, Beta-lactams are ineffective. **Macrolides** (Azithromycin) or Doxycycline are the drugs of choice.

Question 698: Which of the following affects cellular immunity?

A. Cytokines (Correct Answer)
B. Immunoglobulins
C. Gamma-globulins
D. Prostaglandins

Explanation: **Explanation:** Immunity is broadly divided into **Humoral Immunity** (mediated by B-cells and antibodies) and **Cellular Immunity** (mediated by T-cells and their products). **Why Cytokines is the correct answer:** Cellular immunity (Cell-Mediated Immunity or CMI) relies on the activation of T-lymphocytes, macrophages, and NK cells. **Cytokines** (such as Interferon-gamma, IL-2, and TNF) are the primary signaling molecules that coordinate this response. They act as chemical messengers that regulate the growth, differentiation, and activation of effector cells involved in CMI. For example, Th1 cells secrete IFN-γ to activate macrophages to destroy intracellular pathogens. **Analysis of Incorrect Options:** * **B & C. Immunoglobulins / Gamma-globulins:** These are synonymous in this context. Immunoglobulins (Antibodies) are the mediators of **Humoral Immunity**. They are produced by plasma cells (derived from B-cells) and act against extracellular pathogens. They do not directly mediate cellular immunity. * **D. Prostaglandins:** These are lipid compounds derived from arachidonic acid that act as local mediators of inflammation, fever, and pain. While they can modulate immune responses, they are not the primary effectors or regulators of cellular immunity. **High-Yield NEET-PG Pearls:** * **Key CMI Cells:** CD4+ T-helper cells and CD8+ Cytotoxic T-cells. * **Tests for CMI:** Skin tests (e.g., Mantoux/Tuberculin test), Lymphocyte transformation test, and Migration inhibition factor (MIF) test. * **Intracellular Pathogens:** CMI is the primary defense against viruses, fungi, and intracellular bacteria like *M. tuberculosis* and *L. leprae*. * **Cytokine Profile:** Th1 cells produce IL-2 and IFN-γ (promoting CMI), while Th2 cells produce IL-4, IL-5, and IL-10 (promoting Humoral immunity).

Question 699: A 15-year-old male presents with fever (103°F), severe headache, stiff neck, and a petechial rash. A lumbar puncture reveals Gram-negative diplococci (Neisseria meningitidis). Which of the following tests, combining features of gel diffusion and immunoelectrophoresis, is applicable to negatively charged antigens for confirming the causative agent?

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

Explanation: ### Explanation **Correct Answer: B. Counter immunoelectrophoresis (CIE)** **Why it is correct:** Counter immunoelectrophoresis (CIE) is a rapid serological technique that combines the principles of **gel diffusion** and **electrophoresis**. It is specifically designed for **negatively charged antigens** (like the capsular polysaccharides of *N. meningitidis*, *H. influenzae*, and *S. pneumoniae*). In this method, the antigen and antibody are placed in separate wells in an agar gel. An electric current is applied: the negatively charged antigen migrates toward the anode (+), while the antibody moves toward the cathode (-) due to electroendosmosis. They meet in the middle, forming a visible **precipitation line** at the zone of equivalence. Its speed (30–60 minutes) and sensitivity make it ideal for detecting bacterial antigens in CSF. **Why other options are incorrect:** * **A. Coagglutination (COA):** This uses *Staphylococcus aureus* (Cowan 1 strain) coated with Protein A to bind the Fc portion of antibodies. It is an agglutination test, not an electrophoresis-based gel diffusion method. * **C. ELISA:** While highly sensitive for detecting antigens/antibodies, it relies on enzyme-substrate reactions for color change and does not involve the migration of charged particles through a gel under an electric field. * **D. Latex Agglutination (LA):** This involves latex beads coated with specific antibodies. It is a popular bedside test for meningitis but is a simple agglutination reaction, not a combination of gel diffusion and electrophoresis. **High-Yield Clinical Pearls for NEET-PG:** * **CIE Application:** Most useful for detecting capsular antigens in sterile body fluids (CSF, urine, serum). * **N. meningitidis:** The petechial rash (purpura fulminans) is a hallmark sign of meningococcemia. * **Speed:** CIE is significantly faster than standard Ouchterlony double diffusion. * **Sensitivity:** While LA is now more commonly used due to simplicity, CIE remains the classic "textbook" answer for electrophoresis-based antigen detection in meningitis.

Question 700: Allergic rhinitis is which type of hypersensitivity reaction?

A. Type-1 (Correct Answer)
B. Type-2
C. Type-3
D. Type-4

Explanation: **Explanation:** **Allergic rhinitis** is a classic example of **Type-1 (Immediate) Hypersensitivity**. This reaction is mediated by **IgE antibodies** and occurs within minutes of exposure to an allergen (like pollen or dust). Upon first exposure, the individual is sensitized, leading to IgE production which binds to the surface of **mast cells and basophils**. On re-exposure, the allergen cross-links these IgE molecules, triggering **degranulation** and the release of primary mediators like **histamine**, which causes sneezing, rhinorrhea, and nasal congestion. **Why other options are incorrect:** * **Type-2 (Cytotoxic):** Mediated by IgG or IgM against antigens on specific cell surfaces or tissues. Examples include ABO incompatibility, Rh incompatibility, and Myasthenia Gravis. * **Type-3 (Immune-Complex):** Caused by the deposition of antigen-antibody complexes in tissues, leading to complement activation. Examples include SLE, Post-streptococcal glomerulonephritis (PSGN), and Arthus reaction. * **Type-4 (Delayed-type):** A cell-mediated response involving T-lymphocytes (Th1/Th17) rather than antibodies. It typically takes 48–72 hours to manifest. Examples include the Mantoux test, Contact dermatitis, and Lepromin test. **High-Yield Clinical Pearls for NEET-PG:** * **Coombs and Gell Classification:** The standard system used to classify these four types. * **Type-1 Mnemonic:** **"A"** for **A**naphylaxis, **A**topy, and **A**sthma. * **Key Cells:** Mast cells are the central effector cells in Type-1 reactions. * **Late-phase reaction:** Occurs 2–8 hours after the initial Type-1 response, primarily mediated by **eosinophils**.

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Immunology — MCQs

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