Immunology — MCQs
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Which is the primary secretory immunoglobulin?
Which of the following antibacterial substances is present in eggs and saliva?
Which component of the complement system is an anaphylotoxin?
Which type of cells are most dependent on IFN-gamma?
Which of the following is NOT a Type I hypersensitivity reaction?
Interferon is what in nature?
Which immunoglobulin is associated with allergic disorders?
Which type of hypersensitivity reaction mimics Type 5 hypersensitivity?
What part of an antigen attaches to an antibody?
Which method is used to detect antigen-antibody reactions?
Immunology Indian Medical PG Practice Questions and MCQs
Question 361: Which is the primary secretory immunoglobulin?
- A. IgG
- B. IgA (Correct Answer)
- C. IgM
- D. IgE
Explanation: **Explanation:** **IgA (Immunoglobulin A)** is the primary secretory antibody because it is the most abundant immunoglobulin found in mucosal secretions (tears, saliva, colostrum, and gastrointestinal/respiratory tracts). In its secretory form, it exists as a **dimer** held together by a J-chain and a **secretory component**. This secretory component is crucial as it protects the antibody from enzymatic degradation by proteases present in the gut and other secretions, allowing it to provide local "mucosal immunity" by preventing the attachment of pathogens to epithelial surfaces. **Why other options are incorrect:** * **IgG:** This is the most abundant antibody in the **serum** (not secretions) and the only one that crosses the placenta. It is the primary mediator of the secondary immune response. * **IgM:** This is the largest antibody (pentamer) and the first to appear in the **primary immune response**. While it also contains a J-chain, it is not the primary secretory antibody. * **IgE:** This is involved in **Type I hypersensitivity** reactions and provides defense against helminthic parasitic infections. It is found in very low concentrations in the serum. **High-Yield NEET-PG Pearls:** * **Selective IgA Deficiency:** The most common primary immunodeficiency; patients are prone to recurrent sinopulmonary infections and Giardiasis. * **Colostrum:** Rich in IgA, providing passive immunity to the neonate. * **Half-life:** IgG has the longest half-life (approx. 23 days). * **Valency:** Secretory IgA is tetravalent (4 antigen-binding sites).
Question 362: Which of the following antibacterial substances is present in eggs and saliva?
- A. Lysozyme (Correct Answer)
- B. Secretozyme
- C. Albumin
- D. Isozyme
Explanation: **Explanation:** **Lysozyme** (also known as muramidase) is a key component of the innate immune system. It is an enzyme that exerts antibacterial activity by hydrolyzing the **β-1,4 glycosidic bonds** between N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) in the **peptidoglycan** layer of bacterial cell walls. It is found in high concentrations in egg whites and various human secretions, including **saliva, tears, nasal secretions, and sweat**, as well as within the granules of neutrophils. It is particularly effective against Gram-positive bacteria, which have a thick, exposed peptidoglycan layer. **Analysis of Incorrect Options:** * **Secretozyme:** This is a fictitious term and does not represent a recognized biological enzyme or antibacterial substance. * **Albumin:** While the major protein in egg whites (ovalbumin) and human serum, its primary functions are maintaining oncotic pressure and transporting molecules; it does not possess direct enzymatic antibacterial properties. * **Isozyme:** This is a general biochemical term for enzymes that differ in amino acid sequence but catalyze the same chemical reaction. It is not a specific antibacterial substance. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** Lysozyme acts as a "natural antibiotic" by targeting the cell wall. * **Tears:** Lysozyme in tears is the primary reason the ocular surface remains remarkably resistant to infection despite constant exposure. * **Deficiency:** Low levels of lysozyme in secretions can predispose individuals to increased mucosal infections. * **Other Innate Factors:** Remember other substances like **Lactoferrin** (sequesters iron) and **Defensins** (pore-forming peptides) which often appear alongside lysozyme in exam questions regarding innate immunity.
Question 363: Which component of the complement system is an anaphylotoxin?
- A. C3b
- B. C5b-9
- C. C5a (Correct Answer)
- D. C1-3
Explanation: ### Explanation The complement system is a crucial part of innate immunity, consisting of proteins that enhance the ability of antibodies and phagocytic cells to clear pathogens. **1. Why C5a is the Correct Answer:** Anaphylatoxins are small peptide fragments (C3a, C4a, and C5a) released during complement activation. They trigger degranulation of mast cells and basophils, releasing histamine and increasing vascular permeability. Among these, **C5a is the most potent anaphylatoxin**. It also acts as a powerful **chemotactic factor**, recruiting neutrophils and macrophages to the site of inflammation. **2. Analysis of Incorrect Options:** * **A. C3b:** This is primarily an **opsonin**. It coats the surface of pathogens, making them more "tasty" for phagocytes (opsonization). It also helps in the formation of C5 convertase. * **B. C5b-9:** This represents the **Membrane Attack Complex (MAC)**. It forms a pore in the lipid bilayer of the target cell, leading to osmotic lysis and cell death. * **C. C1-3:** This refers to the early components of the Classical pathway. While C3 is cleaved to produce the anaphylatoxin C3a, the complex C1-3 as a whole does not function as an anaphylatoxin. **3. High-Yield Clinical Pearls for NEET-PG:** * **Potency Order:** C5a > C3a > C4a. * **Opsonization:** C3b and iC3b are the major opsonins. * **Chemotaxis:** C5a is the primary complement-derived chemoattractant for neutrophils. * **Deficiency:** Deficiency of C5-C9 (MAC) predisposes individuals to recurrent **Neisserial infections** (Meningitis and Gonorrhea). * **Inhibitor:** **C1 esterase inhibitor** deficiency leads to Hereditary Angioedema.
Question 364: Which type of cells are most dependent on IFN-gamma?
- A. IFN-gamma (Correct Answer)
- B. IL-4
- C. IL-5
- D. IL-6
Explanation: **Explanation:** The question focuses on the cytokine profile of **Th1 cells**. Th1 cells are primarily responsible for cell-mediated immunity and are most dependent on **IFN-gamma** for both their differentiation and their primary effector functions. 1. **Why IFN-gamma is correct:** IFN-gamma acts in a positive feedback loop. It induces the transcription factor **T-bet**, which commits naive T-cells to the Th1 lineage. Once activated, Th1 cells secrete IFN-gamma to activate **macrophages**, enhancing their microbicidal activity (killing of intracellular pathogens like *M. tuberculosis*). 2. **Why the other options are incorrect:** * **IL-4:** This is the signature cytokine for **Th2 cells**. It promotes B-cell differentiation into plasma cells and stimulates the isotype switch to **IgE**. * **IL-5:** Also produced by Th2 cells, its primary role is the activation and chemotaxis of **eosinophils**, crucial in the defense against helminthic infections. * **IL-6:** This is a pro-inflammatory cytokine involved in the acute phase response and the differentiation of **Th17 cells** (along with TGF-beta). **High-Yield Clinical Pearls for NEET-PG:** * **Th1 Profile:** IFN-gamma, IL-2, TNF-beta (Targets: Intracellular pathogens). * **Th2 Profile:** IL-4, IL-5, IL-10, IL-13 (Targets: Extracellular parasites, Allergens). * **Key Transcription Factors:** Th1 = **T-bet**; Th2 = **GATA-3**; Th17 = **ROR-gamma-t**. * **Leprosy Link:** Tuberculoid leprosy (strong CMI) shows a Th1 response, while Lepromatous leprosy (poor CMI) shows a Th2 response.
Question 365: Which of the following is NOT a Type I hypersensitivity reaction?
- A. Casoni's test
- B. Food allergy
- C. Prausnitz-Küstner reaction
- D. Contact dermatitis (Correct Answer)
Explanation: **Explanation:** Hypersensitivity reactions are classified by the Gell and Coombs system based on the immune mechanism involved. **Why Contact Dermatitis is the correct answer:** **Contact dermatitis** is a classic example of **Type IV (Delayed-type) Hypersensitivity**. Unlike Type I, it is mediated by **T-lymphocytes** (CD4+ and CD8+ cells) rather than antibodies. Upon re-exposure to an allergen (like nickel or poison ivy), sensitized T-cells release cytokines that recruit macrophages, leading to inflammation typically 48–72 hours after exposure. **Why the other options are incorrect:** All other options are examples of **Type I (Immediate) Hypersensitivity**, which is mediated by **IgE antibodies** and mast cell degranulation: * **Casoni’s test:** An immediate skin test used to diagnose Hydatid disease. Injection of hydatid fluid causes a wheal-and-flare response within 20 minutes in sensitized individuals. * **Food allergy:** Ingestion of allergens (e.g., peanuts, shellfish) triggers IgE-mediated mast cell activation, leading to symptoms ranging from urticaria to systemic anaphylaxis. * **Prausnitz-Küstner (PK) reaction:** A historical method used to demonstrate "reaginic" (IgE) antibodies. Serum from an allergic person is injected into a healthy person's skin; subsequent challenge at that site produces a local Type I reaction. **High-Yield Clinical Pearls for NEET-PG:** * **Type I:** IgE-mediated; "Immediate"; Examples: Anaphylaxis, Atopy, Bronchial Asthma. * **Type II:** Cytotoxic (IgG/IgM); Examples: Erythroblastosis fetalis, Goodpasture syndrome, Myasthenia gravis. * **Type III:** Immune-complex mediated; Examples: SLE, Arthus reaction, Serum sickness. * **Type IV:** Cell-mediated; "Delayed"; Examples: Mantoux test, Lepromin test, Graft rejection.
Question 366: Interferon is what in nature?
- A. Protein (Correct Answer)
- B. Lipid
- C. Polysaccharides
- D. All of the above
Explanation: **Explanation:** **Interferons (IFNs)** are a group of signaling proteins, specifically categorized as **cytokines**, which are released by host cells in response to the presence of several viruses. Chemically, they are **glycoproteins** (proteins with a carbohydrate side chain), which makes **Option A (Protein)** the correct answer. They play a crucial role in the innate immune response by inducing an "antiviral state" in neighboring uninfected cells. * **Why Option A is correct:** Interferons are encoded by specific genes and synthesized on ribosomes as polypeptide chains. They function by binding to specific cell-surface receptors, triggering a JAK-STAT signaling pathway that leads to the synthesis of antiviral enzymes (like 2',5'-oligoadenylate synthetase and protein kinase R). * **Why Options B & C are incorrect:** While lipids and polysaccharides are essential components of the cell membrane and bacterial cell walls (e.g., LPS), they do not possess the signaling or enzymatic-regulatory functions characteristic of interferons. Interferons are purely proteinaceous in their primary functional structure. **High-Yield Clinical Pearls for NEET-PG:** * **Classification:** * **Type I:** IFN-α (produced by leukocytes) and IFN-β (produced by fibroblasts). Primarily antiviral. * **Type II:** IFN-γ (produced by Th1 cells and NK cells). Primarily immunomodulatory; activates macrophages. * **Mechanism:** They do not kill viruses directly. Instead, they inhibit viral protein synthesis and degrade viral RNA. * **Clinical Use:** Recombinant IFNs are used to treat Hepatitis B, Hepatitis C, Multiple Sclerosis (IFN-β), and Chronic Granulomatous Disease (IFN-γ). * **Species Specificity:** Interferons are highly species-specific (e.g., human interferons work best in human cells).
Question 367: Which immunoglobulin is associated with allergic disorders?
- A. IgG
- B. IgM
- C. IgA
- D. IgE (Correct Answer)
Explanation: **Explanation:** **Correct Answer: D. IgE** IgE is the primary mediator of **Type I Hypersensitivity reactions** (allergic disorders). It has a high affinity for **FcεRI receptors** located on the surface of mast cells and basophils. Upon re-exposure to an allergen, the allergen cross-links the IgE molecules bound to these cells, triggering degranulation and the release of inflammatory mediators like histamine, leukotrienes, and prostaglandins. This leads to clinical manifestations such as asthma, hay fever, urticaria, and anaphylaxis. **Incorrect Options:** * **IgG:** The most abundant antibody in serum. It is responsible for secondary immune responses, opsonization, and Type II/III hypersensitivity. 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 the most effective at complement activation. * **IgA:** The primary secretory immunoglobulin found in colostrum, saliva, tears, and mucosal surfaces. It provides local immunity against respiratory and intestinal pathogens. **NEET-PG High-Yield Pearls:** * **Prausnitz-Küstner (PK) Reaction:** A classic test used to demonstrate IgE-mediated skin sensitivity (historically significant). * **Parasitic Infections:** IgE levels are characteristically elevated in helminthic infections (e.g., *Ascaris*, *Strongyloides*) to facilitate Eosinophil-mediated ADCC. * **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.
Question 368: Which type of hypersensitivity reaction mimics Type 5 hypersensitivity?
- A. Type 1
- B. Type 2 (Correct Answer)
- C. Type 3
- D. Type 4
Explanation: **Explanation:** The correct answer is **Type 2 Hypersensitivity**. **Why Type 2 is correct:** Historically, **Type 5 hypersensitivity** (Stimulatory Hypersensitivity) was classified separately to describe conditions where antibodies bind to cell surface receptors and *stimulate* rather than destroy the cell. The classic example is **Graves' Disease**, where Thyroid Stimulating Immunoglobulins (TSI) mimic TSH. However, modern immunology (Coombs and Gell classification) now categorizes these stimulatory reactions as a **subtype of Type 2 Hypersensitivity**. Since Type 2 involves IgG or IgM antibodies directed against antigens on specific cell surfaces or tissues, the mechanism of receptor-binding fits perfectly within this category. **Why other options are wrong:** * **Type 1 (Immediate):** Mediated by IgE and mast cell degranulation (e.g., Anaphylaxis, Asthma). It does not involve receptor stimulation via autoantibodies. * **Type 3 (Immune-complex):** Involves the deposition of soluble antigen-antibody complexes in tissues (e.g., SLE, Post-streptococcal glomerulonephritis). * **Type 4 (Delayed):** Cell-mediated immunity involving T-lymphocytes and macrophages, not antibodies (e.g., Mantoux test, Contact dermatitis). **NEET-PG High-Yield Pearls:** * **Graves' Disease:** The most common example of "Type 5" (now Type 2 stimulatory). * **Myasthenia Gravis:** Another example where antibodies bind to ACh receptors; however, these are *blocking* antibodies (inhibitory Type 2), whereas Graves' involves *stimulatory* antibodies. * **Mnemonic for Type 2:** "Antibody-Mediated" or "Cytotoxic." Remember: **A**llergic (1), **A**ntibody (2), **I**mmune-Complex (3), **D**elayed (4).
Question 369: What part of an antigen attaches to an antibody?
- A. Epitope (Correct Answer)
- B. Paratope
- C. Idiotype
- D. Allotype
Explanation: ### Explanation **Correct Option: A. Epitope** An **epitope** (also known as the **antigenic determinant**) is the specific chemical group or molecular configuration on the surface of an antigen to which an antibody binds. Antigens are typically large molecules, but the immune system recognizes only these small, discrete sites (usually 5–15 amino acids or sugar residues). An antigen can have multiple epitopes, allowing different antibodies to bind simultaneously. **Analysis of Incorrect Options:** * **B. Paratope:** This is the **antigen-binding site on the antibody**. It is the specific region (located in the hypervariable regions of the Fab fragment) that recognizes and fits into the epitope. A simple mnemonic: **E**pitope is on the **E**nemy (Antigen); **P**aratope is on the **P**rotector (Antibody). * **C. Idiotype:** This refers to the unique set of antigenic determinants found in the variable region (V-region) of an antibody molecule that determines its specificity. * **D. Allotype:** These are antigenic variations in the constant region of antibodies that differ among individuals of the same species due to genetic alleles (e.g., Gm markers on IgG). **High-Yield Clinical Pearls for NEET-PG:** * **Haptens:** These are low-molecular-weight substances that are **antigenic but not immunogenic** on their own. They require a carrier protein to elicit an immune response. * **Adjuvants:** Substances (like Alum) added to vaccines to enhance the immunogenicity of an antigen without being antigenic themselves. * **Valency:** The number of epitopes on an antigen surface. Most natural antigens are multivalent.
Question 370: Which method is used to detect antigen-antibody reactions?
- A. ELISA
- B. Southern Blot
- C. Northern Blot (Correct Answer)
- D. Western Blot
Explanation: **Explanation:** The question asks for a method to detect antigen-antibody reactions. However, there is a technical discrepancy in the provided key: **ELISA (Option A)** and **Western Blot (Option D)** are the standard methods for detecting antigen-antibody interactions. **Northern Blot (Option C)** is traditionally used for RNA detection. If we follow standard medical microbiology: 1. **ELISA (Enzyme-Linked Immunosorbent Assay):** This is the gold standard for detecting antigen-antibody reactions. It uses enzyme-labeled antibodies to quantify specific antigens or antibodies in a patient's serum (e.g., screening for HIV or Hepatitis B). 2. **Western Blot:** This method detects specific **proteins** (antigens) using labeled antibodies after gel electrophoresis. It is the classic confirmatory test for HIV. 3. **Southern Blot:** Used for the detection of specific **DNA** sequences using DNA probes. (Mnemonic: **S**outhern = **D**NA). 4. **Northern Blot:** Used for the detection of specific **RNA** sequences (mRNA) to study gene expression. (Mnemonic: **N**orthern = **R**NA). **Note on the Correct Answer:** While the key indicates Northern Blot, in standard NEET-PG curriculum, Northern Blot detects RNA, not antigen-antibody reactions. If the question intended to identify a "Blotting" technique for proteins, **Western Blot** would be the correct choice. **High-Yield Clinical Pearls for NEET-PG:** * **SNOW DROP Mnemonic:** **S**outhern-**D**NA, **N**orthern-**R**NA, **O**-O, **W**estern-**P**rotein. * **ELISA** is the most common screening test; **Western Blot** is the confirmatory test. * **Eastern Blot** is used to detect post-translational modifications of proteins (carbohydrates/lipids).
Practice by Chapter
Cells and Organs of Immune System
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Innate Immunity
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Adaptive Immunity
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Antigens and Antibodies
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Major Histocompatibility Complex
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Complement System
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Cytokines and Chemokines
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Hypersensitivity Reactions
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Autoimmunity and Autoimmune Diseases
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Immunodeficiency Disorders
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Transplantation Immunology
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Tumor Immunology
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