Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 641: A 25-year-old man is exposed to Ascaris but does not develop clinical signs of infection. Which of the following mechanisms is likely to be responsible for his resistance to infection?

A. Antibody-mediated destruction of worm-infected host cells
B. Cytotoxic T-lymphocyte (CTL) induced apoptosis of worm-infected host cells
C. Complement-mediated lysis of worms attached to host tissues
D. IgE-mediated type I hypersensitivity disrupting worm attachment (Correct Answer)

Explanation: **Explanation:** The body’s primary defense mechanism against large multicellular helminths (like *Ascaris*) is a **Type I Hypersensitivity reaction** mediated by **IgE antibodies** and **Mast cells**. Because helminths are too large to be phagocytosed, the immune system employs "expulsion" tactics. When a person is exposed to *Ascaris*, Th2 cells stimulate B-cells to produce IgE. These IgE antibodies bind to mast cells via FcεRI receptors. Upon re-exposure, the parasite antigens cross-link the IgE, causing mast cell degranulation. This releases potent mediators like **histamine** and **leukotrienes**, which increase intestinal motility (peristalsis) and mucus production. This "weep and sweep" response physically disrupts the worm's attachment and flushes it out of the gastrointestinal tract before it can establish a clinical infection. **Analysis of Incorrect Options:** * **Options A & B:** These mechanisms (ADCC and CTLs) target **intracellular** pathogens by destroying "infected host cells." Helminths are **extracellular** parasites; they do not live inside host cells, making these mechanisms ineffective. * **Option C:** While complement can be activated, most helminths have evolved thick integuments (cuticles) and regulatory proteins that make them highly resistant to the Membrane Attack Complex (MAC)-mediated lysis. **NEET-PG High-Yield Pearls:** * **Key Cells:** Eosinophils are the primary effector cells against helminths; they release **Major Basic Protein (MBP)** via IgE-mediated ADCC to damage the parasite's cuticle. * **Cytokine Profile:** Helminthic infections are characterized by a **Th2 response** (IL-4, IL-5, and IL-13). * **IL-5** is specifically responsible for the eosinophilia commonly seen in these patients.

Question 642: Which of the following statements regarding Cytotoxic T lymphocytes (CTLs) is correct?

A. Recognize the same antigens as CD4 T cells
B. Secrete cytokines that stimulate the differentiation and proliferation of B cells
C. Are important in the control of viral infections (Correct Answer)
D. Most often recognize antigens that have been phagocytosed, degraded, and presented on the surface of an APC

Explanation: **Explanation:** **Cytotoxic T Lymphocytes (CTLs)**, which are CD8+ T cells, play a pivotal role in cell-mediated immunity by identifying and eliminating cells infected with intracellular pathogens, particularly **viruses**, as well as tumor cells. 1. **Why Option C is Correct:** CTLs recognize viral peptides presented by **MHC Class I** molecules on the surface of any nucleated cell. Once activated, they induce apoptosis of the infected cell via the **Perforin-Granzyme pathway** or **Fas-Fas ligand interaction**, effectively halting viral replication and spread. 2. **Why Other Options are Incorrect:** * **Option A:** CTLs (CD8+) recognize antigens presented on **MHC Class I**, whereas CD4+ T cells recognize antigens on **MHC Class II**. They do not recognize the same antigen-MHC complex. * **Option B:** This is the function of **Helper T cells (CD4+)**, specifically the Th2 subset, which secretes IL-4 and IL-5 to stimulate B cell differentiation into plasma cells. * **Option D:** This describes the **Exogenous Pathway** of antigen processing (MHC II), typical for CD4+ T cells. CTLs recognize antigens processed via the **Endogenous Pathway**, where proteins synthesized within the cytosol (like viral proteins) are degraded by proteasomes. **High-Yield Clinical Pearls for NEET-PG:** * **MHC Restriction:** CD8+ = MHC I (Rule of 8: 8 × 1 = 8); CD4+ = MHC II (4 × 2 = 8). * **Markers:** CTLs express CD8, CD3, and the T-cell receptor (TCR). * **Granzymes:** Serine proteases that enter the target cell to activate caspases, leading to programmed cell death.

Question 643: A patient is genetically unable to synthesize J chains. Which immunoglobulins would be affected?

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

Explanation: **Explanation:** The **J chain (Joining chain)** is a small glycoprotein (approx. 15 kDa) required for the polymerization of specific immunoglobulins. It is synthesized by plasma cells and is essential for stabilizing the multimeric structures of **IgM** and **IgA**. * **IgM:** Typically exists as a **pentamer** in serum. The J chain is required to link the five monomeric units together. * **IgA:** While it exists as a monomer in serum, it forms **dimers** in secretions (Secretory IgA). The J chain is vital for dimerization and facilitates the binding of IgA to the secretory component for transport across mucosal surfaces. Therefore, a genetic inability to synthesize J chains would prevent the formation of pentameric IgM and dimeric IgA, severely compromising mucosal immunity and the primary immune response. **Analysis of Incorrect Options:** * **IgG, IgE, and IgD:** These immunoglobulins always function as **monomers**. They do not possess or require a J chain for their structure or function. * **Options A, B, and D:** These are incorrect because they include IgG or IgE, which are unaffected by J chain deficiency. **NEET-PG High-Yield Pearls:** * **Structure:** IgM is a pentamer (contains J chain); IgA is a dimer (contains J chain) or monomer. * **Valency:** Pentameric IgM has a theoretical valency of 10, but an effective valency of 5 due to steric hindrance. * **Mucosal Immunity:** IgA is the most produced antibody in the body, primarily found at mucosal surfaces (tears, saliva, colostrum, GI tract). * **Complement Activation:** IgM is the most potent activator of the classical complement pathway due to its multimeric structure.

Question 644: Memory T cells can be identified by using which of the following markers?

A. CD 45 RA
B. CD 45 RB
C. CD 45 RC
D. CD 45 RO (Correct Answer)

Explanation: **Explanation:** The identification of T cell subsets relies on the expression of different isoforms of the **CD45 molecule** (Leukocyte Common Antigen). CD45 is a tyrosine phosphatase essential for T-cell receptor signaling. **Why CD45RO is correct:** Memory T cells (both CD4+ and CD8+) are characterized by the expression of **CD45RO**. When a "naive" T cell encounters an antigen, it undergoes alternative splicing of the CD45 gene, removing the A, B, and C exons. This results in the shortest isoform, CD45RO. This marker indicates that the cell has been previously activated and has transitioned into a long-lived memory state, allowing for a rapid immune response upon re-exposure to the same pathogen. **Analysis of Incorrect Options:** * **CD45RA:** This is the marker for **Naive T cells** (cells that have not yet encountered their specific antigen). It is the longest isoform containing the 'A' peptide. * **CD45RB and CD45RC:** These isoforms are expressed on various subsets of B cells, naive T cells, and NK cells, but they are not specific markers used to identify the memory T cell population in clinical immunology. **High-Yield Facts for NEET-PG:** * **Naive T cells:** CD45RA+ (Think 'A' for "At rest" or "Antigen-inexperienced"). * **Memory T cells:** CD45RO+ (Think 'O' for "Old" or "Once-activated"). * **CD45** is known as the **Leukocyte Common Antigen (LCA)** and is used in immunohistochemistry to differentiate lymphomas from carcinomas. * **Central Memory T cells (Tcm)** also express **CCR7 and L-selectin (CD62L)**, allowing them to home to lymph nodes, whereas **Effector Memory T cells (Tem)** lack these and circulate in peripheral tissues.

Question 645: The Group A carbohydrate antigen of Streptococcus pyogenes shares antigenic similarity with which human tissue component?

A. Synovial fluid
B. Myocardium
C. Cardiac valves (Correct Answer)
D. Vascular intima

Explanation: **Explanation:** The correct answer is **Cardiac valves**. This phenomenon is a classic example of **molecular mimicry**, where exogenous antigens share structural similarities with host self-antigens, leading to an autoimmune response. 1. **Why Cardiac Valves?** The cell wall of *Streptococcus pyogenes* (Group A Strep) contains a specific **Group A carbohydrate antigen** (N-acetylglucosamine). This antigen shares structural homology with **glycoproteins** found in human **cardiac valves**. Following a pharyngeal infection, the immune system produces antibodies against the bacterial carbohydrate; these antibodies cross-react with the valvular tissue, leading to the inflammatory damage seen in **Acute Rheumatic Fever (ARF)**. 2. **Analysis of Incorrect Options:** * **Myocardium:** While the myocardium is involved in Rheumatic Fever, it is the **M protein** of *S. pyogenes* that mimics myocardial **sarcolemma and myosin**, not the Group A carbohydrate. * **Synovial fluid:** Joint involvement (migratory polyarthritis) in ARF is due to immune complex deposition rather than direct molecular mimicry with the Group A carbohydrate. * **Vascular intima:** Though vasculitis can occur in various streptococcal sequelae, it is not the primary target of the Group A carbohydrate cross-reactivity. **High-Yield Clinical Pearls for NEET-PG:** * **M Protein:** The most important virulence factor of *S. pyogenes*; it mimics **myocardial sarcolemma**. * **Hyaluronic Acid Capsule:** Mimics human **synovial fluid/connective tissue** (making the bacteria non-immunogenic). * **Cytoplasmic Membrane:** Shares antigens with the **subthalamic and caudate nuclei**, leading to Sydenham’s Chorea. * **Jones Criteria:** Used for the diagnosis of ARF (Major: Joint, Carditis, Nodules, Erythema marginatum, Sydenham chorea).

Question 646: Which immunoglobulin has the highest molecular weight?

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

Explanation: **Explanation:** The molecular weight of an immunoglobulin is primarily determined by its structural configuration (monomer vs. polymer) and the number of amino acids in its heavy chains. **Why IgM is correct:** IgM is the largest antibody, often referred to as the **"Millionaire Molecule"** due to its high molecular weight of approximately **900,000 Daltons (900 kDa)**. In its secreted form, IgM exists as a **pentamer** (five Y-shaped units) held together by disulfide bonds and a polypeptide called the **J-chain** (Joining chain). This pentameric structure gives it 10 antigen-binding sites, making it highly efficient in agglutination and complement activation. **Why the other options are incorrect:** * **IgG (150 kDa):** The most abundant antibody in serum. It exists as a monomer and has the lowest molecular weight among the options, allowing it to cross the placenta. * **IgE (190 kDa):** Exists as a monomer. While its heavy chain (epsilon) is slightly longer than IgG’s, it remains significantly smaller than the pentameric IgM. * **IgA (160–385 kDa):** Exists as a monomer in serum but as a **dimer** in secretions (tears, saliva, colostrum). Even as a dimer, its weight (~385 kDa) does not exceed that of IgM. **High-Yield NEET-PG Pearls:** * **IgM:** First antibody to appear in response to an antigen (acute infection) and the first to be synthesized by the fetus. * **Valency:** IgM has a theoretical valency of 10, but an effective valency of 5 due to steric hindrance. * **Sedimentation Coefficient:** IgM is 19S (the highest), while IgG is 7S. * **Intravascular Distribution:** Due to its large size, IgM is confined primarily to the intravascular compartment.

Question 647: Which of the following statements is true about isotypic variation?

A. These result due to subtle amino acid changes resulting from allelic differences.
B. These result due to changes in amino acids in the heavy and light chains at the variable region.
C. Changes in the heavy and light chains in the constant region are responsible for the class and subclass of immunoglobulins. (Correct Answer)
D. These are areas in an antigen that bind specifically to an antibody.

Explanation: **Explanation:** The classification of immunoglobulins is based on three types of variations: **Isotypes, Allotypes, and Idiotypes.** **Why Option C is Correct:** **Isotypic variation** refers to the differences in the **constant (C) regions** of the heavy and light chains that are present in all healthy members of a species. The heavy chain constant region determines the **class** (IgG, IgA, IgM, IgE, IgD) and **subclass** (e.g., IgG1, IgG2) of the antibody. For example, the presence of a 'gamma' ($\gamma$) heavy chain defines the IgG isotype. Similarly, light chains are classified into 'kappa' ($\kappa$) or 'lambda' ($\lambda$) isotypes based on their constant region sequences. **Analysis of Incorrect Options:** * **Option A (Allotypes):** This describes **Allotypic variation**. These are subtle amino acid differences in the constant region resulting from different **alleles** of the same gene. Unlike isotypes, allotypes vary between individuals of the same species (e.g., Gm markers on IgG). * **Option B (Idiotypes):** This describes **Idiotypic variation**. These changes occur in the **variable (V) regions** (specifically the hypervariable regions) of the heavy and light chains. They determine the unique antigen-binding specificity of an individual antibody molecule. * **Option D (Epitopes):** This describes an **Epitope** (antigenic determinant), which is the specific part of an antigen that is recognized and bound by the antibody's paratope. **High-Yield NEET-PG Pearls:** * **Isotype:** Constant region; defines Class/Subclass; same in all individuals of a species. * **Allotype:** Constant region; due to Alleles; differs between individuals. * **Idiotype:** Variable region; defines Specificity; unique to a single clone of B-cells. * **Memory Trick:** **I**sotype = **S**pecies-specific; **A**llotype = **A**llele-specific; **I**diotype = **I**ndividual antibody-specific.

Question 648: Which cell type produces IL-I?

A. Macrophage (Correct Answer)
B. Helper T lymphocytes
C. B cells
D. Cytotoxic T-cells

Explanation: **Explanation:** **Interleukin-1 (IL-1)** is a key pro-inflammatory cytokine that serves as a primary mediator of the innate immune response. 1. **Why Macrophages are correct:** Macrophages (and monocytes) are the **primary cellular source** of IL-1. Upon activation by pathogen-associated molecular patterns (PAMPs) like Endotoxin (LPS), macrophages secrete IL-1. Its chief functions include inducing fever (by acting on the hypothalamus), stimulating the liver to produce acute-phase reactants (CRP, Fibrinogen), and activating T-cells and vascular endothelium. 2. **Why other options are incorrect:** * **Helper T lymphocytes (CD4+):** These cells primarily produce **IL-2, IL-4, IL-5, and IFN-gamma**. While they are the targets of IL-1 (which acts as a co-stimulator for T-cell activation), they do not produce it. * **B cells:** Their primary role is antigen presentation and antibody production. While they can produce some cytokines (like IL-6 or IL-10), they are not a major source of IL-1. * **Cytotoxic T-cells (CD8+):** These cells are involved in direct cell killing via perforins and granzymes and produce cytokines like **IFN-gamma**, but not IL-1. **High-Yield Clinical Pearls for NEET-PG:** * **Endogenous Pyrogen:** IL-1 is known as the "Endogenous Pyrogen" because it stimulates PGE2 synthesis in the anterior hypothalamus to raise the body's temperature set-point. * **IL-1 Family:** It exists in two forms, **IL-1α** (cell-bound) and **IL-1β** (secreted). * **Inflammasome:** The maturation of IL-1β requires the activation of the **Caspase-1** enzyme within a protein complex called the Inflammasome. * **Synergy:** IL-1 often works synergistically with **TNF-α** and **IL-6** to mediate systemic inflammatory response syndrome (SIRS).

Question 649: Which of the following statements best describes a key feature of innate immunity?

A. It improves with repeated exposure to a given antigen
B. It requires antigen processing for activation
C. It is not associated with primary immune deficiency
D. It includes interaction between pattern recognition receptors on phagocytes and pathogen-associated molecular patterns (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. It includes interaction between pattern recognition receptors on phagocytes and pathogen-associated molecular patterns** **Why it is correct:** Innate immunity is the body's first line of defense and is characterized by its **non-specific** nature. It relies on germline-encoded receptors called **Pattern Recognition Receptors (PRRs)**, such as Toll-like receptors (TLRs), located on cells like macrophages and neutrophils. These PRRs recognize highly conserved molecular structures found on microbes, known as **Pathogen-Associated Molecular Patterns (PAMPs)** (e.g., Lipopolysaccharide or Flagellin). This interaction allows for immediate recognition and response without prior exposure. **Why the other options are incorrect:** * **Option A:** This describes **Adaptive Immunity**. Innate immunity lacks memory; its response remains the same regardless of how many times it encounters the same pathogen. * **Option B:** Antigen processing and presentation (via MHC molecules) are hallmarks of **T-cell activation** in adaptive immunity. Innate cells respond directly to PAMPs without needing complex processing. * **Option C:** Innate immunity **is** associated with primary immune deficiencies. Examples include **Chronic Granulomatous Disease (CGD)** (defect in NADPH oxidase) and **Chédiak-Higashi syndrome** (defect in lysosomal trafficking). **NEET-PG High-Yield Pearls:** * **Speed:** Innate immunity acts within minutes to hours; Adaptive takes days to weeks. * **Toll-Like Receptors (TLRs):** TLR-4 recognizes Gram-negative LPS (Endotoxin); TLR-2 recognizes Gram-positive Peptidoglycan. * **Components:** Includes physical barriers (skin), chemical barriers (lysozyme), cells (NK cells, neutrophils), and the Complement system (Alternative and Lectin pathways). * **Diversity:** Innate immunity has limited diversity (fixed receptors), whereas adaptive immunity has high diversity due to genetic recombination.

Question 650: Which type of hypersensitivity reaction can occur when a single dose of antigen acts as both the sensitizing and shocking dose?

A. Anaphylaxis
B. Arthus reaction
C. Serum sickness (Correct Answer)
D. Contact dermatitis

Explanation: **Explanation:** The correct answer is **Serum sickness**, which is a classic example of a **Type III (Immune-complex mediated) hypersensitivity reaction**. **Why Serum Sickness is the correct answer:** In most hypersensitivity reactions, an initial "sensitizing dose" is required to prime the immune system, followed by a "shocking dose" later to trigger the reaction. However, in serum sickness, a **single large dose** of a foreign antigen (e.g., horse serum, certain antibiotics) persists in the body long enough to act as both. 1. **Sensitization:** During the first 5–10 days, the body starts producing antibodies against the circulating antigen. 2. **Shocking:** While the original antigen is still present in the circulation, these newly formed antibodies bind to it, forming **soluble immune complexes**. These complexes deposit in blood vessels, joints, and kidneys, activating complement and causing systemic symptoms (fever, rash, polyarthritis, and glomerulonephritis). **Analysis of Incorrect Options:** * **A. Anaphylaxis (Type I):** Requires a prior sensitizing dose to produce IgE. The "shocking dose" must occur upon re-exposure to trigger mast cell degranulation. * **B. Arthus Reaction (Type III):** This is a **localized** immune complex reaction. It occurs in an individual who *already* has high levels of circulating antibodies when the antigen is injected intradermally. * **D. Contact Dermatitis (Type IV):** A delayed-type hypersensitivity mediated by T-cells. It requires a sensitization phase (hapten binding) and a subsequent challenge phase. **NEET-PG High-Yield Pearls:** * **Serum Sickness** is characterized by the triad of **fever, rash (urticaria), and arthralgia**. * It is a **systemic** Type III reaction, whereas the Arthus reaction is **local**. * Common triggers today include **Antithymocyte globulin (ATG)**, **Rituximab**, and **Penicillin**. * **Complement levels (C3, C4)** are typically **decreased** during the acute phase due to consumption.

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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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Immunology — MCQs

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