Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 561: Interleukin 1 (IL-1) is a potent cytokine. It is best described by which one of the following statements?

A. It can be produced by natural killer cells (Correct Answer)
B. It exerts its effects on T and B cells as a costimulator
C. It is multimeric and consists of more than one protein
D. Synthesis of IL-1 is inhibited in activated macrophages

Explanation: ### Explanation **Interleukin-1 (IL-1)** is a key pro-inflammatory cytokine primarily associated with the innate immune response. **Why Option A is Correct:** While **monocytes and macrophages** are the primary sources of IL-1, it is also produced by a variety of other cells, including **Natural Killer (NK) cells**, B cells, dendritic cells, endothelial cells, and epithelial cells. In the context of NEET-PG, it is crucial to remember that IL-1 production is not exclusive to myeloid cells. **Analysis of Incorrect Options:** * **Option B:** This statement is partially true but less accurate than A. IL-1 acts as a co-stimulator for T-cell activation (enhancing IL-2 production); however, its primary role is as an endogenous pyrogen and a mediator of acute-phase responses. * **Option C:** IL-1 is not multimeric. It exists in two distinct **monomeric** forms, **IL-1α and IL-1β**, which are encoded by separate genes but bind to the same receptor. * **Option D:** This is factually incorrect. Synthesis of IL-1 is significantly **increased and induced** in activated macrophages, especially upon exposure to PAMPs (like LPS) via Toll-like receptors. --- ### High-Yield Clinical Pearls for NEET-PG: * **The "Fever" Cytokine:** IL-1 is a potent **endogenous pyrogen**. It acts on the anterior hypothalamus to increase prostaglandin E2 (PGE2) synthesis, raising the thermoregulatory set point. * **The Inflammasome Connection:** IL-1β is synthesized as an inactive precursor (pro-IL-1β). It requires cleavage by **Caspase-1** within the **NLRP3 inflammasome** complex to become biologically active. * **Acute Phase Response:** Along with IL-6 and TNF-α, IL-1 stimulates the liver to produce acute-phase reactants (e.g., CRP, Fibrinogen). * **Clinical Correlation:** **Anakinra** is a recombinant IL-1 receptor antagonist used in the treatment of Rheumatoid Arthritis and Cryopyrin-Associated Periodic Syndromes (CAPS).

Question 562: Who discovered phagocytosis?

A. Metchnikoff (Correct Answer)
B. Ehrlich
C. Ruska
D. Pasteur

Explanation: **Explanation:** **1. Correct Answer: A. Metchnikoff** Elie Metchnikoff, a Russian zoologist, discovered **phagocytosis** in 1882 while studying starfish larvae. He observed that specialized cells (phagocytes) could engulf and digest foreign particles. This discovery laid the foundation for **Cellular Immunity**. For his work, he shared the Nobel Prize in 1908 with Paul Ehrlich. **2. Analysis of Incorrect Options:** * **B. Ehrlich:** Paul Ehrlich is known as the "Father of Chemotherapy." He proposed the **Side-Chain Theory** to explain antibody formation and discovered Salvarsan (for syphilis). He is the pioneer of **Humoral Immunity**. * **C. Ruska:** Ernst Ruska designed and built the first **Electron Microscope** in 1931, which revolutionized microbiology by allowing the visualization of viruses and internal cell structures. * **D. Pasteur:** Louis Pasteur is the "Father of Medical Microbiology." His contributions include the **Germ Theory of Disease**, the process of Pasteurization, and the development of vaccines for Anthrax, Cholera, and **Rabies**. **3. NEET-PG High-Yield Pearls:** * **Cellular vs. Humoral:** Remember Metchnikoff = Cellular Immunity; Ehrlich = Humoral Immunity. * **The "Starfish" Experiment:** Metchnikoff’s discovery began by inserting a rose thorn into a starfish larva and observing the cellular reaction. * **Opsonization:** While Metchnikoff discovered the "eating" action, the term "Opsonin" (substances that coat bacteria to make them more "tasty" to phagocytes) was coined by Almroth Wright. * **Phagocytic Cells:** In humans, the primary professional phagocytes are **Neutrophils** (microphages) and **Macrophages**.

Question 563: Which of the following cell surface markers are associated with T helper cells?

A. CD2- CD3- CD4- CD8- TCR- cell
B. CD2+ CD3+ CD4- CD8- TCR+ cell
C. CD2+ CD3+ CD4+ CD8- TCR+ cell (Correct Answer)
D. CD2+ CD3+ CD4- CD8+ TCR+ cell

Explanation: **Explanation:** The identification of T-lymphocyte subsets is based on the presence of specific **Cluster of Differentiation (CD)** markers. All mature T-cells are characterized by the presence of **CD2** (an adhesion molecule/LFA-2), **CD3** (part of the T-cell receptor complex for signal transduction), and a functional **TCR** (T-cell receptor). **1. Why Option C is Correct:** T helper (Th) cells are defined as **CD4+** cells. They recognize antigens presented by **MHC Class II** molecules. Therefore, a mature T helper cell will express **CD2+, CD3+, CD4+, and TCR+**, while being negative for CD8. **2. Analysis of Incorrect Options:** * **Option A:** This represents a "null cell" or a very early pro-T cell (double negative) found in the thymus before maturation. It lacks the essential markers of a functional T-lymphocyte. * **Option B:** This describes a "double-negative" T-cell. While these exist in small numbers in the peripheral blood (often γδ T-cells), they do not function as classical T helper cells. * **Option D:** This describes a **Cytotoxic T-lymphocyte (CTL)**. These cells are **CD8+** and recognize antigens presented by **MHC Class I** molecules. **Clinical Pearls for NEET-PG:** * **CD3** is the most specific marker for all T-cells. * **CD4:CD8 Ratio:** In a healthy individual, the normal ratio is approximately **2:1**. * **HIV Pathogenesis:** HIV selectively infects CD4+ cells by binding to the CD4 molecule (using gp120), leading to a depletion of T helper cells and a reversal of the CD4:CD8 ratio. * **MHC Restriction Rule:** CD4 x II = 8 and CD8 x I = 8 (The product of the CD marker and the MHC class always equals 8).

Question 564: Which of the following antigens is found within the nuclei of infected hepatocytes and not usually in the peripheral circulation in Hepatitis B infection?

A. HBeAg
B. HBcAg (Correct Answer)
C. Anti-HBc
D. HBsAg

Explanation: **Explanation:** The correct answer is **HBcAg (Hepatitis B core Antigen)**. **Why HBcAg is the correct answer:** HBcAg is a particulate antigen that forms the inner nucleocapsid of the Hepatitis B virus. During viral replication, it is sequestered within the **nuclei of infected hepatocytes**. Crucially, HBcAg is enveloped by the surface protein (HBsAg) before being released into the bloodstream. Because it is "hidden" inside the complete virion (Dane particle), **free HBcAg is not detectable in the peripheral circulation**. Its presence can only be demonstrated via immunofluorescence in liver biopsy specimens. **Analysis of Incorrect Options:** * **HBeAg (Option A):** This is a soluble protein secreted by infected cells. It is found in the serum and serves as a marker of high viral replication and infectivity. * **Anti-HBc (Option B):** This is an antibody produced by the host against the core antigen. It is easily detectable in the serum and is a key marker for diagnosing past or current infection (the "Window Period" marker). * **HBsAg (Option D):** This is the surface antigen found on the outer envelope. It is the first marker to appear in the blood during an acute infection and is the primary screening tool. **High-Yield Clinical Pearls for NEET-PG:** * **The "Window Period":** The time between the disappearance of HBsAg and the appearance of Anti-HBs. During this time, **IgM Anti-HBc** is the only reliable serological marker. * **HBeAg vs. Anti-HBe:** HBeAg indicates high infectivity; its disappearance and the appearance of Anti-HBe (seroconversion) indicate a lower risk of transmission. * **HBcAg** is the only HBV antigen that does not circulate freely in the blood.

Question 565: Which of the following is not a phagocytic cell?

A. Macrophages
B. Kupffer cells
C. NK cells (Correct Answer)
D. Neutrophils

Explanation: **Explanation:** The core concept tested here is the distinction between **phagocytes** (cells that ingest and destroy pathogens) and **cytotoxic lymphocytes** (cells that induce apoptosis in target cells). **1. Why NK cells are the correct answer:** Natural Killer (NK) cells are large granular lymphocytes belonging to the innate immune system. Unlike macrophages or neutrophils, NK cells are **not phagocytic**. Instead, they function through **direct cytotoxicity**. They identify virally infected or tumor cells (often via "missing self" MHC-I recognition) and release **perforins and granzymes** to induce programmed cell death (apoptosis). **2. Why the other options are incorrect:** * **Neutrophils (Option D):** These are the "first responders" of the immune system and are professional phagocytes. They utilize oxygen-dependent (respiratory burst) and oxygen-independent mechanisms to kill ingested microbes. * **Macrophages (Option A):** These are mature forms of monocytes found in tissues. They are highly efficient professional phagocytes and also act as Antigen Presenting Cells (APCs). * **Kupffer cells (Option B):** These are specialized, **fixed macrophages** located in the sinusoids of the liver. As part of the Reticuloendothelial System (RES), their primary role is the phagocytosis of pathogens and debris from the portal circulation. **Clinical Pearls for NEET-PG:** * **Professional Phagocytes:** Neutrophils, Monocytes, Macrophages, and Dendritic cells. * **NK Cell Markers:** CD16 (FcγRIII) and CD56 are characteristic surface markers. * **Mnemonic for Fixed Macrophages:** * Liver: Kupffer cells * CNS: Microglia * Lungs: Alveolar macrophages (Dust cells) * Skin: Langerhans cells * Bone: Osteoclasts

Question 566: Which antibody has the maximum concentration in gastrointestinal secretions?

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

Explanation: **Explanation:** The correct answer is **IgA**. **Why IgA is correct:** Immunoglobulin A (IgA) is the primary antibody involved in **mucosal immunity**. In the body, it exists in two forms: a monomer in the serum and a **dimer** in secretions. Secretory IgA (sIgA) is specifically adapted to survive in harsh environments like the gastrointestinal (GI) tract. It contains a **J-chain** that holds the dimer together and a **secretory component** (derived from epithelial cells) that protects the antibody from proteolytic enzymes present in GI secretions. It acts by "immune exclusion," preventing the attachment of pathogens to mucosal surfaces. **Why other options are incorrect:** * **IgG:** This is the most abundant antibody in the **serum** (75-80%) and provides systemic immunity. It is the only antibody that crosses the placenta but is not the primary defender in GI secretions. * **IgM:** This is the largest antibody (pentamer) and the first to appear in a primary immune response. While it also contains a J-chain and can be secreted onto mucosal surfaces in IgA-deficient individuals, its concentration is significantly lower than IgA. * **IgD:** Found primarily on the surface of B-cells as a receptor; it has no known major role in mucosal secretions. **NEET-PG High-Yield Pearls:** * **Most abundant Ig overall:** IgG (Serum) vs. **Most produced Ig daily:** IgA (due to the vast surface area of mucous membranes). * **Selective IgA Deficiency:** The most common primary immunodeficiency; patients often present with recurrent GI infections (e.g., *Giardia*) and respiratory infections. * **Breast Milk:** IgA is the predominant antibody in colostrum, providing passive mucosal immunity to the neonate.

Question 567: A patient presents with generalized edema, sweating, flushing, tachycardia, and fever after a bee sting. What type of hypersensitivity reaction is most likely responsible?

A. T cell mediated cytotoxicity
B. IgE mediated reaction (Correct Answer)
C. IgG mediated reaction
D. IgA mediated hypersensitivity reaction

Explanation: **Explanation:** The clinical presentation of generalized edema, flushing, tachycardia, and sweating following a bee sting is a classic manifestation of **Anaphylaxis**, which is a **Type I Hypersensitivity Reaction**. **1. Why Option B is Correct:** Type I hypersensitivity is an **IgE-mediated reaction**. Upon re-exposure to an allergen (bee venom), specific IgE antibodies already bound to the surface of **mast cells and basophils** cause cross-linking. This triggers immediate degranulation and the release of potent vasoactive mediators like **histamine**, leukotrienes, and prostaglandins. These mediators cause systemic vasodilation (flushing, tachycardia), increased vascular permeability (edema), and smooth muscle contraction. **2. Why the other options are incorrect:** * **Option A (T cell mediated):** This refers to **Type IV (Delayed) Hypersensitivity**. It involves sensitized T-lymphocytes and typically occurs 48–72 hours after exposure (e.g., Mantoux test or contact dermatitis), not immediately. * **Option C (IgG mediated):** IgG is primarily involved in **Type II** (Cytotoxic, e.g., Autoimmune hemolytic anemia) and **Type III** (Immune-complex, e.g., SLE or Serum Sickness) reactions. While IgG can sometimes play a role in subacute reactions, the rapid systemic collapse seen here is classically IgE-driven. * **Option D (IgA mediated):** IgA is involved in mucosal immunity. It is not a primary mediator of systemic hypersensitivity reactions. **High-Yield Clinical Pearls for NEET-PG:** * **Gell and Coombs Classification:** Remember the mnemonic **ACID** (Type I: **A**naphylactic/Atopic; Type II: **C**ytotoxic; Type III: **I**mmune Complex; Type IV: **D**elayed). * **Drug of Choice:** Intramuscular **Epinephrine (1:1000)** is the first-line treatment for anaphylaxis. * **Diagnostic Marker:** Serum **tryptase** levels can be measured shortly after the event to confirm mast cell degranulation.

Question 568: How many four-peptide subunits does IgM have?

A. 3
B. 4
C. 5 (Correct Answer)
D. 6

Explanation: **Explanation:** **Correct Answer: C (5)** The basic structural unit of any immunoglobulin (monomer) consists of **four peptide chains**: two identical heavy (H) chains and two identical light (L) chains, linked by disulfide bonds. **IgM** is unique because it primarily exists as a **pentamer** in its secreted form. This means it is composed of **five** of these four-peptide subunits, which are arranged in a circular fashion and held together by disulfide bonds and a specialized polypeptide called the **J chain (Joining chain)**. Therefore, a single IgM molecule contains a total of 20 peptide chains (10 heavy and 10 light). **Analysis of Incorrect Options:** * **Option A (3) & B (4):** No naturally occurring human immunoglobulin exists as a trimer or tetramer in its primary secreted state. * **Option D (6):** While some older literature or specific experimental conditions might mention hexameric IgM (which lacks a J chain), the standard physiological form tested in medical exams is the pentamer. **High-Yield Clinical Pearls for NEET-PG:** * **Valency:** Although IgM is a pentamer (10 antigen-binding sites), its effective valency is often considered **5** due to steric hindrance. * **Molecular Weight:** IgM is the largest antibody ("Millionaire molecule"), making it the most effective at agglutination and complement fixation (via the classical pathway). * **First Responder:** IgM is the first antibody produced in a primary immune response and the first to appear in phylogeny and ontogeny. * **Intravascular Distribution:** Due to its large size, IgM is largely confined to the intravascular compartment.

Question 569: What is the primary immunoglobulin mediating the secondary immune response?

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

Explanation: **Explanation:** The primary immune response occurs upon the first exposure to an antigen, characterized by a lag phase and the initial production of **IgM**. However, the **secondary immune response** (anamnestic response) occurs upon re-exposure to the same antigen. This response is faster, more intense, and longer-lasting due to the activation of memory B cells. **IgG** is the predominant immunoglobulin in this phase because of class switching and affinity maturation, providing high-affinity protection. **Analysis of Options:** * **IgG (Correct):** It is the most abundant antibody in serum and the primary mediator of the secondary response. It is the only antibody that crosses the placenta. * **IgM (Incorrect):** This is the first antibody produced during the **primary immune response**. Its presence indicates an acute or recent infection. * **IgA (Incorrect):** This is the primary secretory immunoglobulin, found in colostrum, saliva, and tears. It provides mucosal immunity but is not the hallmark of the systemic secondary response. * **IgE (Incorrect):** This antibody is primarily involved in Type I hypersensitivity (allergic) reactions and provides defense against helminthic parasitic infections. **High-Yield NEET-PG Pearls:** * **Memory:** The secondary response is mediated by Memory B cells. * **Avidity vs. Affinity:** While IgM has higher valency (pentamer), IgG produced in the secondary response has higher **affinity** due to somatic hypermutation. * **Diagnostic Marker:** High IgM levels suggest **acute infection**, while high IgG levels suggest **past infection or chronic state**. * **Half-life:** IgG has the longest half-life (approx. 23 days) among all immunoglobulins.

Question 570: Which theory is most commonly accepted for antibody production?

A. Direct template theory
B. Indirect template theory
C. Natural selection theory
D. Clonal selection theory (Correct Answer)

Explanation: **Explanation:** The **Clonal Selection Theory**, proposed by **Frank Macfarlane Burnet** in 1957, is the most widely accepted model for antibody production and immune response. **1. Why Clonal Selection Theory is Correct:** This theory states that the body contains a vast repertoire of pre-existing B-lymphocytes, each carrying a unique surface receptor for a specific antigen. When an antigen enters the body, it "selects" the specific lymphocyte that matches its shape. This selected cell is then activated to proliferate (clone itself) and differentiate into: * **Plasma cells:** Which secrete large quantities of antibodies specific to that antigen. * **Memory cells:** Which provide long-term immunity. **2. Why Other Options are Incorrect:** * **Direct Template Theory (Pauling):** Suggested that antigens act as a physical mold or "template" around which a generic antibody protein folds. This was proven wrong because protein folding is determined by amino acid sequences, not external templates. * **Indirect Template Theory:** Proposed that the antigen modifies the DNA or protein-synthesis machinery to produce specific antibodies. This contradicts the central dogma of molecular biology. * **Natural Selection Theory (Jerne):** Suggested that all possible antibodies already circulate in the blood, and the antigen simply binds to one and transports it to a cell for replication. While it introduced the idea of pre-existing diversity, it lacked the cellular mechanism of clonal expansion. **High-Yield Clinical Pearls for NEET-PG:** * **Key Scientist:** Burnet (Nobel Prize winner). * **Self-Tolerance:** Clonal selection explains how the body avoids autoimmunity; clones that react to "self-antigens" are deleted during embryonic development (**Clonal Deletion**). * **Monoclonal Antibodies:** The concept of "one cell, one antibody" is the basis for producing monoclonal antibodies (Hybridoma technology).

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Complement System

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Cytokines and Chemokines

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Hypersensitivity Reactions

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Immunology — MCQs

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