Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 851: Both antibody-dependent and independent complement pathways converge on which complement component?

A. C5
B. C3 (Correct Answer)
C. Clq
D. C8

Explanation: **Explanation:** The complement system consists of three distinct pathways: the **Classical** (antibody-dependent), the **Alternative** (antibody-independent), and the **Lectin** pathway. 1. **Why C3 is correct:** All three pathways converge at the formation of **C3 convertase**. This enzyme cleaves C3 into C3a (anaphylatoxin) and C3b (opsonin). C3 is the most abundant complement protein in the serum and represents the "final common pathway" for the initial activation steps. Once C3 is cleaved, the pathways merge into the terminal lytic sequence. 2. **Why other options are incorrect:** * **C1q:** This is the recognition unit specifically for the **Classical pathway** only. It binds to the Fc portion of IgM or IgG. It is not involved in the Alternative or Lectin pathways. * **C5:** While all pathways eventually lead to the activation of C5 to form the Membrane Attack Complex (MAC), the actual point of convergence where the distinct activation cascades meet is at the **C3 level**. C5 activation occurs downstream of C3. * **C8:** This is a component of the terminal Membrane Attack Complex (C5b-9). It is involved much later in the cascade, long after the pathways have converged. **High-Yield Clinical Pearls for NEET-PG:** * **C3 deficiency:** The most severe complement deficiency, leading to recurrent pyogenic infections (e.g., *S. pneumoniae*) and Type III hypersensitivity reactions. * **C1 esterase inhibitor deficiency:** Leads to **Hereditary Angioedema** (due to overproduction of bradykinin). * **C5-C9 deficiency:** Specifically predisposes patients to recurrent **Neisseria** infections (meningitis and gonorrhea). * **Opsonization:** C3b is the primary opsonin; **C3b** binds to **B**acteria (easy mnemonic).

Question 852: What is true about a silent mutation in a gene?

A. No change in mRNA
B. No change in amino acid sequence in protein (Correct Answer)
C. No expression of protein
D. No change in the expression of protein

Explanation: ### Explanation **Concept Overview** A **silent mutation** is a type of point mutation where a single nucleotide base is substituted for another, but the resulting codon still codes for the same amino acid. This phenomenon is possible due to the **degeneracy (redundancy) of the genetic code**, where multiple codons can specify a single amino acid (e.g., GAA and GAG both code for Glutamic Acid). **Why Option B is Correct** The hallmark of a silent mutation is that while the DNA and mRNA sequences change, the **primary structure of the protein (amino acid sequence) remains identical**. Since the protein remains unchanged, its folding and function typically remain normal. **Analysis of Incorrect Options** * **Option A:** Inaccurate. A mutation by definition involves a change in the DNA sequence, which is then transcribed into a **changed mRNA sequence**. * **Option C:** Inaccurate. The gene is still transcribed and translated; the protein is expressed, just with the original amino acid sequence. * **Option D:** While often true, this is not the *definition* of a silent mutation. Some silent mutations can affect mRNA stability or splicing, potentially altering the *amount* of protein expressed, but the defining feature is the lack of change in the amino acid sequence itself. **High-Yield Clinical Pearls for NEET-PG** * **Wobble Hypothesis:** Silent mutations usually occur at the **third position** of the codon. * **Synonymous Mutation:** Another term for a silent mutation. * **Contrast with Missense:** A mutation that results in a *different* amino acid (e.g., Sickle Cell Anemia: Glutamic acid → Valine). * **Contrast with Nonsense:** A mutation that creates a premature **stop codon** (UAG, UAA, UGA), leading to a truncated, non-functional protein.

Question 853: Which of the following is not a chemokine?

A. IL-8
B. IL-1
C. Histamine (Correct Answer)
D. Eotaxin

Explanation: ### Explanation **Chemokines** are a specific sub-family of cytokines characterized by their ability to induce **chemotaxis** (directed migration) in responsive cells, such as neutrophils, monocytes, and lymphocytes. They are small signaling proteins (8–10 kDa) usually classified into four groups based on the spacing of their cysteine residues: CC, CXC, C, and CX3C. **Why Histamine is the Correct Answer:** **Histamine** is a **vasoactive amine**, not a chemokine. It is pre-formed and stored in the granules of mast cells and basophils. Its primary functions include vasodilation, increasing vascular permeability, and smooth muscle contraction (bronchoconstriction). While it plays a major role in acute inflammation and Type I hypersensitivity, it does not belong to the cytokine/chemokine protein family. **Analysis of Incorrect Options:** * **IL-8 (CXCL8):** This is the "prototypical" chemokine. It is a CXC chemokine produced by macrophages and endothelial cells, acting as a potent chemoattractant and activator for **neutrophils**. * **IL-1:** While primarily known as a pro-inflammatory cytokine (inducing fever and acute-phase reactants), certain isoforms and its role in inducing other chemokines often lead to its classification within broader inflammatory recruitment discussions. However, in many classical texts, IL-1 is considered a primary cytokine that *triggers* chemokine production. (Note: In some competitive contexts, IL-1 is the "distractor," but compared to Histamine, it is much closer to the cytokine signaling family). * **Eotaxin (CCL11):** This is a CC chemokine specifically responsible for the recruitment of **eosinophils** to inflammatory sites, particularly in allergic reactions and parasitic infections. **High-Yield Clinical Pearls for NEET-PG:** * **IL-8** is the most important chemokine for **Neutrophils**. * **Eotaxin** is the most important for **Eosinophils**. * **CCR5** (a chemokine receptor) acts as a co-receptor for **HIV** entry into macrophages. * Chemokines are structurally defined by **Cysteine residues**.

Question 854: An IgG2 molecule is composed of which of the following?

A. One alpha, one gamma, and two kappa chains
B. One gamma1 chain and two kappa chains
C. Two gamma1 chains and one kappa and one lambda chain
D. Two gamma2 chains and two kappa chains (Correct Answer)

Explanation: ### Explanation **1. Why the Correct Answer is Right:** An Immunoglobulin (Ig) molecule is a Y-shaped heterodimer consisting of **two identical heavy chains** and **two identical light chains**, linked by disulfide bonds. * **Heavy Chains:** The type of heavy chain determines the class (isotype) of the antibody. IgG uses **gamma ($\gamma$)** chains. Since the question specifies the **IgG2** subclass, it must contain two **$\gamma$2** heavy chains. * **Light Chains:** There are two types of light chains: **kappa ($\kappa$)** and **lambda ($\lambda$)**. A single antibody molecule must contain either two $\kappa$ or two $\lambda$ chains; it can never have one of each. Therefore, an IgG2 molecule is composed of **two $\gamma$2 chains and two $\kappa$ (or two $\lambda$) chains**. Option D is the only choice that follows this structural rule. **2. Analysis of Incorrect Options:** * **Option A:** Alpha ($\alpha$) chains are found in IgA, not IgG. Furthermore, an antibody cannot have only one heavy chain; they always exist in pairs. * **Option B:** An antibody must have two heavy chains to maintain its basic monomeric structure. A single $\gamma$1 chain would result in an incomplete, non-functional molecule. * **Option C:** This violates the rule of **Isotypic Exclusion**. A single immunoglobulin molecule cannot contain both a $\kappa$ and a $\lambda$ light chain simultaneously. **3. NEET-PG Clinical Pearls & High-Yield Facts:** * **Heavy Chain Isotypes:** IgA ($\alpha$), IgD ($\delta$), IgE ($\epsilon$), IgG ($\gamma$), IgM ($\mu$). * **Light Chain Ratio:** In humans, the normal $\kappa$:$\lambda$ ratio is **2:1**. A significant shift in this ratio (e.g., in Multiple Myeloma) indicates monoclonal proliferation. * **IgG Subclasses:** IgG1 is the most abundant; **IgG2** is specifically associated with the immune response against **capsular polysaccharides** (e.g., *S. pneumoniae*). * **Placental Transfer:** IgG is the only antibody class that crosses the placenta (IgG2 crosses the least efficiently compared to IgG1, 3, and 4).

Question 855: Which of the following is NOT part of the lymphoreticular system?

A. T-cells
B. B-cells
C. Platelets (Correct Answer)
D. Macrophages

Explanation: The **lymphoreticular system** (also known as the Reticuloendothelial System or Mononuclear Phagocyte System) consists of a network of cells and organs responsible for immune surveillance, phagocytosis, and antigen presentation. It primarily includes lymphoid cells (lymphocytes) and phagocytic cells derived from the bone marrow. ### Why Platelets is the Correct Answer: **Platelets (Option C)** are anucleated cell fragments derived from megakaryocytes. Their primary function is **hemostasis** (blood clotting) and maintaining vascular integrity. While they play a minor role in inflammatory signaling, they are not considered part of the lymphoreticular system, which is defined by its roles in immune defense and the clearance of particulate matter. ### Explanation of Incorrect Options: * **T-cells (Option A):** These are the primary mediators of cell-mediated immunity. They originate in the bone marrow and mature in the thymus, forming a core component of the lymphoid part of the system. * **B-cells (Option B):** These are responsible for humoral immunity (antibody production). They are essential lymphoid cells that populate the lymph nodes and spleen. * **Macrophages (Option D):** These are the "professional phagocytes" of the system. Whether circulating as monocytes or fixed in tissues (e.g., Kupffer cells in the liver, Alveolar macrophages in the lungs), they are the functional backbone of the reticuloendothelial system. ### NEET-PG High-Yield Pearls: * **Components of the Lymphoreticular System:** Includes the spleen, lymph nodes, thymus, bone marrow, and the Mononuclear Phagocyte System (MPS). * **Tissue-Specific Macrophages (High Yield):** * Liver: **Kupffer cells** * CNS: **Microglia** * Skin: **Langerhans cells** * Bone: **Osteoclasts** * Kidney: **Mesangial cells** * **Primary Function:** To remove "non-self" antigens, aged erythrocytes (primarily in the spleen), and cellular debris.

Question 856: What is a characteristic of natural killer (NK) cells?

A. MHC restricted
B. Antibody-dependent
C. Null cells (Correct Answer)
D. B-lymphocytes

Explanation: **Explanation:** **Natural Killer (NK) cells** are a type of cytotoxic lymphocyte critical to the innate immune system. They are traditionally classified as **"Null cells"** because they lack the characteristic surface markers of both T-cells (TCR/CD3) and B-cells (surface immunoglobulins). While they originate from the common lymphoid progenitor, they do not undergo thymic maturation or gene rearrangement. * **Why Option C is correct:** NK cells are defined as large granular lymphocytes that are **CD3 negative** and **CD16/CD56 positive**. Since they do not express the antigen-specific receptors found on B or T cells, they are historically termed "Null cells." * **Why Option A is incorrect:** Unlike T-cells, NK cells are **MHC-unrestricted**. They do not require the presentation of antigens via MHC molecules; in fact, they preferentially kill cells that have "downregulated" or missing MHC-I (the "missing self" hypothesis), a common tactic used by viruses and tumors to evade T-cells. * **Why Option B is incorrect:** While NK cells can participate in Antibody-Dependent Cellular Cytotoxicity (ADCC) via their CD16 receptor (FcγRIII), their *primary* defining characteristic and innate killing mechanism are **antibody-independent**. * **Why Option D is incorrect:** NK cells are a distinct lineage from B-lymphocytes; B-cells are part of adaptive immunity and produce antibodies. **High-Yield Clinical Pearls for NEET-PG:** * **Markers:** CD56 (adhesion) and CD16 (receptor for Fc portion of IgG). * **Function:** First line of defense against **viral infections** and **tumor surveillance**. * **Cytokines:** Their activity is significantly enhanced by **IL-2 and IL-12**. * **Mechanism:** They induce apoptosis in target cells using **perforins and granzymes**.

Question 857: Which of the following is true about active immunity?

A. Less effective
B. Can be given in an immunodeficient state
C. Immunological memory present (Correct Answer)
D. No lag period

Explanation: **Explanation:** Active immunity occurs when the body’s own immune system is stimulated to produce antibodies and specialized lymphocytes (T-cells) following exposure to an antigen (either via natural infection or vaccination). **Why Option C is Correct:** The hallmark of active immunity is the development of **immunological memory**. Upon initial exposure, memory B and T cells are generated. If the pathogen enters the body again, these cells recognize it immediately, leading to a faster, more robust, and long-lasting secondary immune response. **Analysis of Incorrect Options:** * **A. Less effective:** Active immunity is generally **more effective** and durable than passive immunity, often providing protection for years or even a lifetime. * **B. Can be given in an immunodeficient state:** Active immunization (especially **live vaccines**) is often **contraindicated** in immunodeficient individuals because their immune system cannot mount an effective response, and live pathogens may cause disseminated disease. * **D. No lag period:** Active immunity has a significant **lag period** (usually 5–14 days) while the body undergoes clonal expansion and antibody synthesis. In contrast, passive immunity provides immediate protection. **NEET-PG High-Yield Pearls:** * **Active vs. Passive:** Active = "Self-made" (Slow but long-lasting); Passive = "Borrowed" (Immediate but short-lived). * **Negative Phase:** A transient decrease in circulating antibodies immediately following a booster dose of an antigen in active immunity. * **Combined Immunization:** Giving both active and passive immunity simultaneously at different sites (e.g., Tetanus toxoid + TIG for a dirty wound, or Rabies vaccine + RIG).

Question 858: Surface immunoglobulin is found on which cell type?

A. T-cell
B. B-cell (Correct Answer)
C. NK cell
D. Plasma cell

Explanation: **Explanation:** **B-cells** are characterized by the presence of **Surface Immunoglobulins (sIg)**, which function as the **B-cell Receptor (BCR)**. These membrane-bound antibodies (primarily IgM and IgD) allow B-cells to recognize and bind directly to specific antigens. This binding is the primary signal required for B-cell activation and subsequent differentiation. **Analysis of Options:** * **A. T-cells:** These cells do not possess surface immunoglobulins. Instead, they use **T-cell Receptors (TCR)** to recognize antigens, and only when the antigen is presented by Major Histocompatibility Complex (MHC) molecules. * **C. NK cells:** Natural Killer cells are part of the innate immune system. They lack antigen-specific receptors like sIg or TCR; instead, they use "killer-cell immunoglobulin-like receptors" (KIRs) to monitor MHC-I expression on target cells. * **D. Plasma cells:** These are the terminally differentiated forms of B-cells. While they are "antibody factories," they **lose their surface immunoglobulins** to focus entirely on the synthesis and secretion of soluble antibodies into the circulation. **High-Yield NEET-PG Pearls:** * **B-cell Markers:** CD19, CD20, CD21 (receptor for EBV), and CD22. * **Mature B-cells:** Express both **surface IgM and IgD**. * **Memory B-cells:** Express surface IgG, IgA, or IgE (post-class switching). * **Mnemonic:** B-cells = **B**-cell receptor = **B**ound immunoglobulin. Plasma cells = **P**our out antibodies (secretory, not surface-bound).

Question 859: What is the primary function of IgA?

A. Acts as a mucosal barrier against infection (Correct Answer)
B. Circulating antibody
C. Kills virus-infected cells
D. Activates macrophages

Explanation: **Explanation:** **Immunoglobulin A (IgA)** is the second most common serum Ig but the most abundant antibody in the body's secretions. Its primary role is to provide **mucosal immunity**. 1. **Why Option A is correct:** IgA exists primarily as a **dimer** in secretions (tears, saliva, colostrum, and mucus of the respiratory, GI, and GU tracts). It contains a **J-chain** and a **secretory component** that protects it from enzymatic degradation. Its main function is **immune exclusion**: it prevents the attachment and colonization of pathogens (bacteria and viruses) to mucosal surfaces, effectively acting as a "first-line" barrier. 2. **Why other options are incorrect:** * **Option B:** While IgA is found in serum, **IgG** is the primary circulating antibody (comprising 75-80% of total serum Ig) and is responsible for systemic secondary immune responses. * **Option C:** Killing virus-infected cells is the primary role of **Cytotoxic T-cells (CD8+)** and **Natural Killer (NK) cells**, not antibodies directly. * **Option D:** Macrophage activation is primarily mediated by **Interferon-gamma (IFN-γ)** produced by Th1 cells. **High-Yield Clinical Pearls for NEET-PG:** * **Selective IgA Deficiency:** The most common primary immunodeficiency. Patients are often asymptomatic but may present with recurrent sinopulmonary infections or giardiasis. * **Breastfeeding:** Colostrum is rich in IgA, providing passive mucosal immunity to the neonate. * **Alternative Pathway:** IgA can activate the complement system via the **alternative pathway** (unlike IgG and IgM, which use the classical pathway). * **Nephrology Link:** IgA nephropathy (Berger’s disease) involves IgA1 deposition in the renal mesangium.

Question 860: Injectable tetanus toxoid (TT) is an example of:

A. Active immunity (Correct Answer)
B. Passive immunity
C. Native immunity
D. Reaction immunity

Explanation: **Explanation:** **1. Why Active Immunity is Correct:** Active immunity occurs when the body’s own immune system is stimulated to produce antibodies and memory cells following exposure to an antigen. **Tetanus Toxoid (TT)** is a modified bacterial toxin that has lost its toxicity but retained its antigenicity. When injected, it triggers a primary immune response, leading to the production of protective antitoxins by the host’s B-lymphocytes. This provides long-lasting protection and is the basis for the National Immunization Schedule. **2. Why Other Options are Incorrect:** * **Passive Immunity:** This involves the direct administration of pre-formed antibodies (e.g., Tetanus Immunoglobulin - TIG). It provides immediate but temporary protection and does not stimulate the recipient's immune system. * **Native (Innate) Immunity:** This is the non-specific, first line of defense present from birth (e.g., skin, mucosal barriers, phagocytes). It does not involve memory or specific recognition of the tetanus toxin. * **Reaction Immunity:** This is not a standard immunological term used to classify types of immunity. **3. High-Yield Clinical Pearls for NEET-PG:** * **Combined Prophylaxis:** In a non-immunized person with a tetanus-prone wound, both **TT (Active)** and **TIG (Passive)** are given simultaneously at different sites. This is called **Active-Passive Immunization**. * **Toxoids:** Other examples of toxoid vaccines include Diphtheria and Botulinum. * **Memory:** Active immunity is characterized by a "lag period" but results in **immunological memory**, whereas passive immunity has no lag period but no memory. * **Pregnancy:** Two doses of TT (now replaced by Td - Tetanus and adult Diphtheria) are given to pregnant women to prevent Neonatal Tetanus via transplacental transfer of IgG.

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Cells and Organs of Immune System

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Innate 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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Immunology — MCQs

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