Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 951: Most effective bactericidal system within phagocytes is-

A. Cationic basic protein mediated
B. Reactive oxygen metabolite mediated (Correct Answer)
C. Lysozyme mediated
D. Lactoferrin mediated

Explanation: ***Reactive oxygen metabolite mediated*** - The production of **reactive oxygen metabolites** (like superoxide, hydrogen peroxide, and hydroxyl radicals) through the **respiratory burst** is a highly potent mechanism for killing phagocytosed bacteria. - These highly reactive molecules cause **oxidative damage** to bacterial components, leading to their degradation and death. *Cationic basic protein mediated* - **Cationic proteins** (e.g., defensins) have antimicrobial properties by damaging bacterial membranes, but they are generally less potent than reactive oxygen species in overall bacterial killing within phagocytes. - While important, they contribute to a broader array of antimicrobial mechanisms but are not considered the *most effective* single system. *Lysozyme mediated* - **Lysozyme** primarily targets bacterial **peptidoglycan**, breaking down bacterial cell walls, especially in gram-positive bacteria. - It is an important antimicrobial enzyme, but its effectiveness is limited against many gram-negative bacteria with outer membranes and it is generally less destructive than the radical-forming reactive oxygen species. *Lactoferrin mediated* - **Lactoferrin** primarily acts by **chelating iron**, which is an essential nutrient for bacterial growth, thereby inhibiting bacterial proliferation. - While important for bacteriostasis, its direct bactericidal activity is often limited compared to the direct damaging effects of reactive oxygen species.

Question 952: Which of the following is a function of MHC class I and II?

A. Antibody class switching
B. Signal transduction in T cell
C. Antigen presentation to T cell (Correct Answer)
D. Increase the secretion of cytokine

Explanation: ***Antigen presentation to T cell*** - **MHC class I** molecules present **endogenous antigens** to **CD8+ cytotoxic T cells**, leading to the destruction of infected or cancerous cells. - **MHC class II** molecules present **exogenous antigens** to **CD4+ helper T cells**, which then coordinate the immune response. *Antibody class switching* - This process is primarily influenced by **cytokines** secreted by helper T cells, rather than direct MHC function. - While T cells interact with B cells to facilitate class switching, the direct role is not performed by the MHC molecules themselves. *Signal transduction in T cell* - **T cell receptor (TCR)** and associated co-receptors (like **CD3 complex**) are responsible for signal transduction upon antigen recognition. - MHC molecules present the antigen, but they do not directly mediate the intracellular signaling cascades within the T cell. *Increase the secretion of cytokine* - Cytokine secretion is a downstream effect of T cell activation, which occurs after successful antigen presentation by MHC molecules. - MHC molecules' direct function is presentation, not the direct increase in cytokine secretion.

Question 953: Post zone phenomenon is due to

A. Antigen excess (Correct Answer)
B. Antibody excess
C. Haptens
D. Balanced Antigen and Antibody Levels

Explanation: ***Antigen excess*** - The **postzone phenomenon** occurs when there is **excess antigen** relative to antibody in the reaction mixture - This leads to formation of **small, soluble antigen-antibody complexes** that remain in solution and do not precipitate effectively - Results in a **false-negative** or weak reaction despite the presence of both antigen and antibody - Seen in the **descending limb** of the precipitin curve *Antibody excess* - This causes the **prozone phenomenon**, not postzone - Occurs when excess antibody prevents optimal lattice formation - Results in false-negative reactions but due to **antibody excess** rather than antigen excess *Haptens* - Haptens are small molecules that can bind antibodies but are **not immunogenic alone** - Require carrier proteins to elicit immune responses - Not directly related to zone phenomena in precipitation reactions *Balanced Antigen and Antibody Levels* - Represents the **zone of equivalence** - Produces **optimal precipitation** with formation of large, insoluble immune complexes - This is the ideal condition for maximal precipitation, opposite of postzone phenomenon

Question 954: IL-2 is secreted by?

A. Helper T-cells (Correct Answer)
B. Neutrophils
C. NK cells
D. Macrophages

Explanation: ***Helper T-cells*** - **Helper T-cells** (CD4+ T-cells) are the **primary source of IL-2** upon activation by antigen presentation. - **IL-2** acts as a **T-cell growth factor**, essential for the clonal expansion of antigen-specific T-cells and immune response amplification. - Activated **CD8+ cytotoxic T-cells** also produce IL-2, though in smaller amounts. *Neutrophils* - **Neutrophils** are phagocytic cells primarily involved in acute inflammation and bacterial killing. - They mainly produce **chemokines** and **pro-inflammatory cytokines** like IL-8 and IL-1β, but do not secrete significant amounts of IL-2. *NK cells* - **Natural Killer (NK) cells** are part of the innate immune system and are crucial for targeting virus-infected and tumor cells. - While activated NK cells can produce small amounts of IL-2, they are primarily **IL-2 responders** rather than major producers. - NK cells predominantly secrete **IFN-gamma** and **TNF-alpha** upon activation. *Macrophages* - **Macrophages** are antigen-presenting cells that phagocytose pathogens and cellular debris. - They predominantly secrete **pro-inflammatory cytokines** such as TNF-alpha, IL-1, IL-6, and IL-12, rather than IL-2.

Question 955: Caspase involved in activation of IL-1 is which of the following?

A. Caspase 5
B. Caspase 1 (Correct Answer)
C. Caspase 8
D. Caspase 3

Explanation: ***Caspase 1*** - **Caspase 1** (also known as interleukin-1 beta converting enzyme or ICE) is the primary caspase responsible for the proteolytic cleavage and activation of pro-IL-1β and pro-IL-18 into their mature, active forms. - This activation occurs within the **inflammasome complex**, a multiprotein oligomer that assembles in response to various pathogens and danger signals. *Caspase 5* - While **Caspase 5** is an inflammatory caspase, similar to Caspase 1, it primarily functions in the direct activation of pro-IL-1β in certain contexts, particularly in response to *Gram-negative bacteria* through the non-canonical inflammasome. - However, **Caspase 1** is the canonical and most well-known activator of IL-1 in the classical inflammasome pathway. *Caspase 8* - **Caspase 8** is a key **initiator caspase** in the extrinsic pathway of apoptosis, activated by death receptors like Fas and TNF receptors. - Its primary role is in **apoptotic signaling** and it is not directly involved in the proteolytic activation of IL-1. *Caspase 3* - **Caspase 3** is a major **effector caspase** in both the intrinsic and extrinsic pathways of apoptosis. - It executes apoptosis by cleaving numerous cellular substrates and is not directly involved in the **processing of cytokines** like IL-1.

Question 956: MHC II is associated with:-

A. Red blood cells
B. Antigen presenting cells (Correct Answer)
C. Platelets
D. Epithelial cells

Explanation: ***Antigen presenting cells*** - **MHC II (Major Histocompatibility Complex class II)** molecules are primarily expressed on the surface of professional **antigen-presenting cells (APCs)**. - APCs, such as **macrophages**, **dendritic cells**, and **B lymphocytes**, use MHC II to present **extracellularly derived antigens** to **CD4+ T helper cells**. *Red blood cells* - **Red blood cells (RBCs)** are anucleated and lack MHC molecules entirely. - Their primary function is **oxygen transport**, not immune cell communication. *Platelets* - **Platelets** are cell fragments involved in **hemostasis** (blood clotting). - They do not express MHC class II molecules as they are not involved in antigen presentation. *Epithelial cells* - Most **epithelial cells** primarily express **MHC class I** molecules to present **intracellular antigens** to **CD8+ cytotoxic T cells**. - They do not typically express MHC class II unless under specific inflammatory conditions, and even then, not as their primary function.

Question 957: In chronic allergy, which Ig is more persistent in the body?

A. Ig A
B. Ig E (Correct Answer)
C. Ig G
D. Ig M

Explanation: ***Ig E*** - **IgE** is the primary antibody involved in **allergic reactions**, binding to receptors on **mast cells** and **basophils** to trigger histamine release. - In chronic allergy, sustained exposure to allergens leads to continuous production of IgE, making it a **persistent** and dominant immunoglobulin in the allergic response. *Ig A* - **IgA** is mainly found in **mucosal secretions**, such as tears, saliva, and gut, protecting against pathogens at these sites. - While important for immunity, IgA does not play a direct role in the **immediate hypersensitivity reactions** characteristic of chronic allergies. *Ig G* - **IgG** is the most abundant antibody in serum, providing **long-term immunity** against pathogens through neutralization, opsonization, and complement activation. - Though present, IgG is not the **primary mediator** of the **allergic response** in chronic allergy, instead often associated with protective immunity or certain non-IgE mediated hypersensitivities. *Ig M* - **IgM** is the first antibody produced during a **primary immune response** and is effective at activating the complement system. - It is predominantly found in the bloodstream and functions as a **short-term defender**, but it is not directly involved in the pathogenesis or persistence of chronic allergies.

Question 958: Choose the correct option regarding graft rejection.

A. CD4 and CD8 both play a role in graft rejection (Correct Answer)
B. None of the options
C. CD8 only plays a role in graft rejection
D. CD4 only plays a role in graft rejection

Explanation: ***CD4 and CD8 both play a role in graft rejection*** - **CD4+ T cells** (helper T cells) recognize donor MHC class II molecules and differentiate into effector cells that produce cytokines, promoting inflammation and activating other immune cells involved in rejection - **CD8+ T cells** (cytotoxic T lymphocytes, CTLs) recognize donor MHC class I molecules and directly kill donor cells in the graft, leading to tissue destruction - Both T cell subsets are crucial for initiating and mediating different aspects of the immune response against transplanted organs *CD8 only plays a role in graft rejection* - This is incorrect because while **CD8+ T cells** are vital for direct cytotoxicity, **CD4+ T cells** are also essential for orchestrating the overall immune response - **CD4+ T cells** provide help to B cells and CD8+ T cells, and their cytokines can also directly injure graft tissue *CD4 only plays a role in graft rejection* - This is incorrect because although **CD4+ T cells** are critical for initiating and amplifying the immune response through cytokine production and activation of other cells, **CD8+ T cells** are directly responsible for killing graft cells - Both cell types contribute significantly to the complex pathophysiology of graft rejection

Question 959: Which immune response is primarily responsible for controlling herpes simplex virus (HSV) infection during latency?

A. Cell-mediated immunity (Correct Answer)
B. Humoral immunity
C. Complement activation
D. Natural killer cell activity

Explanation: ***Cell-mediated immunity*** - **T lymphocytes**, particularly **CD8+ cytotoxic T cells**, are crucial for controlling HSV during latency by recognizing and eliminating reactivating infected cells. - This response prevents the virus from replicating and causing overt symptoms, maintaining the **latent state** in neuronal ganglia. *Humoral immunity* - While **antibodies** (humoral immunity) can prevent initial infection and reduce viral spread during active outbreaks, they are less effective at clearing already established latent infections within host cells. - Antibodies primarily target **extracellular virus particles** and are not as effective against virus hidden inside cells during latency. *Complement activation* - The **complement system** is an important part of innate immunity that helps clear pathogens and damaged cells. - However, HSV has evolved mechanisms to evade complement, and complement activation itself is not the primary mechanism for maintaining viral latency. *Natural killer cell activity* - **Natural killer (NK) cells** are innate immune cells that can kill virus-infected cells, especially early in infection before adaptive immunity is fully active. - While NK cells contribute to initial containment, **adaptive cell-mediated immunity** (T cells) plays the dominant role in controlling latent HSV.

Question 960: Type IV hypersensitivity reactions involve which of the following immune cells?

A. T cells (Correct Answer)
B. B cells
C. Mast cells
D. Eosinophils

Explanation: ***T cells*** - Type IV hypersensitivity, also known as **delayed-type hypersensitivity**, is primarily mediated by **antigen-specific T cells**, especially **CD4+ T helper cells** and **CD8+ cytotoxic T cells**. - These T cells recognize antigens presented by **MHC class I or II molecules**, leading to the release of **cytokines** that activate macrophages and other immune cells, causing tissue damage. *B cells* - B cells are primarily involved in **humoral immunity** by producing **antibodies**, which mediate **Type I, II, and III hypersensitivity reactions**, but not Type IV. - While they can present antigens to T cells, their direct role in the effector phase of Type IV reactions is negligible. *Mast cells* - Mast cells are crucial effector cells in **Type I hypersensitivity reactions** (immediate hypersensitivity), where they release **histamine** and other mediators upon activation by IgE. - They are not directly involved in the cell-mediated immune responses characteristic of Type IV reactions. *Eosinophils* - Eosinophils are typically associated with **allergic reactions** (often Type I) and **parasitic infections**, where they release cytotoxic granules. - While they can be recruited to sites of inflammation, they are not the primary immune cells mediating the delayed hypersensitivity response in Type IV reactions.

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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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