Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 971: Which antibody is most effective in precipitating soluble antigens in immunological reactions?

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

Explanation: ***IgM*** - **IgM** is a pentameric antibody with **10 antigen-binding sites**, making it the most effective immunoglobulin for precipitating soluble antigens. - Its **high valency** allows rapid and efficient cross-linking of soluble antigens to form large **antigen-antibody lattices** that precipitate out of solution. - IgM is the **first antibody produced** in immune responses and is highly efficient at forming immune complexes even at low concentrations. - Its multivalent structure enables it to create stable precipitates more effectively than bivalent antibodies. *IgG* - **IgG** is a monomeric antibody with only **2 antigen-binding sites**, making it less efficient than IgM at cross-linking soluble antigens. - While IgG can participate in precipitation reactions, it requires **higher concentrations** and more time to form precipitates compared to IgM. - IgG is the most abundant antibody in serum and plays important roles in opsonization, complement fixation, and antibody-dependent cellular cytotoxicity, but is not the most effective at precipitation. *IgA* - **IgA** primarily functions in **mucosal immunity** and exists as a monomer in serum and a dimer in secretions. - Its main role is to **prevent pathogen adherence** to mucosal surfaces rather than precipitating soluble antigens in systemic immunological reactions. *IgD* - **IgD** is found predominantly on the surface of **naive B lymphocytes** as an antigen receptor. - Present in very low concentrations in serum, it has **no significant role** in precipitating soluble antigens or forming immune complexes.

Question 972: Which of the following is the most characteristic feature of MHC class II molecules?

A. Play a role in helper T-cell activation.
B. Found on antigen-presenting cells. (Correct Answer)
C. Present in all nucleated cells.
D. Not involved in innate immunity.

Explanation: ***Found on antigen-presenting cells.*** - **MHC class II molecules** are **characteristically** and **primarily** expressed on professional **antigen-presenting cells (APCs)**: **macrophages**, **dendritic cells**, and **B lymphocytes**. - This **restricted distribution** distinguishes them from **MHC class I molecules**, which are present on nearly all nucleated cells. - Their presence on APCs enables presentation of **exogenous antigens** to **CD4+ helper T cells**. *Play a role in helper T-cell activation.* - While this statement is **true**, it describes the **function** rather than the **characteristic distribution** of MHC class II molecules. - This function is a **consequence** of their presence on APCs, not their defining characteristic. - Many molecules play roles in T-cell activation, but the **restricted expression on APCs** is what specifically characterizes MHC class II. *Present in all nucleated cells.* - This describes **MHC class I molecules**, not MHC class II. - **MHC class I** presents **endogenous antigens** to **cytotoxic T cells (CD8+ T cells)**. - The **ubiquitous distribution** of MHC class I contrasts with the **restricted APC distribution** of MHC class II. *Not involved in innate immunity.* - While this statement is technically **true** (MHC class II is part of **adaptive immunity**), it is stated in the **negative** and describes what MHC class II does **not** do. - This is not a **defining characteristic** compared to their specific cellular distribution. - Many adaptive immune components are "not involved in innate immunity," making this non-specific.

Question 973: What is the normal value of the CD4:CD8 ratio in a healthy individual?

A. 0.5
B. 1
C. 1.5
D. 2 (Correct Answer)

Explanation: ***Correct: 2*** - In a **healthy individual**, the number of **CD4+ T cells** typically outnumbers **CD8+ T cells** by approximately **two to one**. - This ratio is crucial for maintaining proper immune surveillance and is often monitored in conditions like **HIV** infection. - The normal reference range is approximately **1.5-2.5**, with **2:1 being the typical value**. *Incorrect: 0.5* - A ratio of **0.5** indicates that **CD8+ T cells** are present at twice the concentration of **CD4+ T cells**. - This is an **inverted ratio** and can be indicative of immune dysfunction or specific disease states, such as **HIV progression** or **chronic viral infections**. *Incorrect: 1* - A ratio of **1** implies an **equal number** of **CD4+ and CD8+ T cells**. - While not severely inverted, it still represents a shift from the normal healthy state and warrants further investigation. *Incorrect: 1.5* - A ratio of **1.5** is closer to the normal range but suggests a **slightly reduced dominance** of **CD4+ T cells** over CD8+ T cells. - This value might be seen in certain conditions or early stages of immune perturbations, but it's at the lower end of the normal range rather than the typical value.

Question 974: Which of the following statements about Toll-like receptors is false?

A. Regulation of calcium channel (Correct Answer)
B. Activation of immune system
C. Activation of transcription factors
D. Recognize microbial molecules

Explanation: ***Regulation of calcium channel*** - Toll-like receptors (TLRs) are primarily involved in **innate immunity** and pathogen recognition, not in the direct regulation of **calcium channels**. - Their signaling pathways typically lead to the activation of **transcription factors** and the production of inflammatory mediators, rather than ion channel modulation. *Activation of immune system* - TLRs play a crucial role in initiating the **innate immune response** by recognizing conserved microbial components. - Their activation leads to the downstream signaling events that trigger the production of **cytokines**, chemokines, and other immune mediators. *Activation of transcription factors* - Upon ligand binding, TLRs initiate intracellular signaling cascades that converge on the activation of several **transcription factors**, such as **NF-κB** and AP-1. - These transcription factors then translocate to the nucleus to induce the expression of genes involved in host defense and inflammation. *Recognize microbial molecules* - TLRs are a class of **pattern recognition receptors (PRRs)** that specifically recognize pathogen-associated molecular patterns (PAMPs). - Examples of microbial molecules recognized by TLRs include **lipopolysaccharide (LPS)** from gram-negative bacteria, flagellin, and viral nucleic acids.

Question 975: Which of the following cells are professional antigen-presenting cells that extensively express Toll-like receptors to recognize pathogen-associated molecular patterns?

A. Macrophages
B. Dendritic cells (Correct Answer)
C. Natural killer cells
D. Endothelial cells

Explanation: ***Dendritic cells*** - **Dendritic cells** are **professional antigen-presenting cells (APCs)** that express a wide array of Toll-like receptors (TLRs) to recognize various pathogen-associated molecular patterns (PAMPs). - TLR activation in dendritic cells leads to their maturation and robust co-stimulatory molecule expression, making them the **most potent APCs** for initiating adaptive immune responses. - They serve as the critical bridge between innate and adaptive immunity. *Macrophages* - **Macrophages** also express extensive TLRs (TLR1-9) and are professional APCs of the innate immune system. - While macrophages are excellent at TLR-mediated pathogen recognition and phagocytosis, **dendritic cells are superior at migrating to lymph nodes and activating naive T cells**, making them the primary initiators of adaptive immunity. - Both cell types express TLRs extensively, but dendritic cells have specialized trafficking and T cell activation capabilities. *Natural killer cells* - **Natural killer (NK) cells** are lymphocytes of the innate immune system that recognize and kill infected or cancerous cells. - They express different receptors including **activating and inhibitory receptors** (KIRs, NCRs) and have limited TLR expression compared to professional APCs. - NK cells rely more on cytokines and direct cell contact for activation rather than TLR signaling. *Endothelial cells* - **Endothelial cells** can express some TLRs (particularly TLR2, TLR4) in response to inflammatory stimuli to regulate **vascular permeability** and **leukocyte extravasation**. - However, they are not professional immune cells and express TLRs at much lower levels than dendritic cells or macrophages. - Their primary function is maintaining vascular integrity, not comprehensive pathogen recognition.

Question 976: Lymphotactin is which type of chemokine?

A. C (Correct Answer)
B. CC
C. CX3C
D. CXC

Explanation: ***C*** - **Lymphotactin**, also known as **XCL1**, is a chemokine belonging to the C chemokine subfamily. - This subfamily is characterized by having **only one cysteine residue** at the N-terminus, distinguishing it from other chemokine subfamilies. *CC* - **CC chemokines** (or β-chemokines) have **two adjacent cysteine residues** at their N-terminus. - Examples include **MCP-1** and **MIP-1α**, which primarily attract monocytes and macrophages. *CX3C* - **CX3C chemokines** possess a unique motif with **three amino acids separating the first two cysteine residues** (C-X3-C). - **Fractalkine (CX3CL1)** is the sole member of this subfamily, acting as both an adhesion molecule and a chemoattractant. *CXC* - **CXC chemokines** (or α-chemokines) have **one amino acid separating the first two cysteine residues** at their N-terminus (C-X-C). - Examples include **IL-8** and **IP-10**, which primarily attract neutrophils and lymphocytes, respectively.

Question 977: CD4 is associated with which HLA?

A. HLA class I
B. HLA class II (Correct Answer)
C. HLA class III
D. None of the options

Explanation: ***HLA class II*** - **CD4** co-receptors on T-helper cells bind to **MHC class II** molecules, which present exogenous antigens. - This interaction is crucial for the activation of helper T-cells and the initiation of a **humoral immune response**. *HLA class I* - **CD8** co-receptors on cytotoxic T-cells bind to **MHC class I** molecules, which present endogenous antigens. - This interaction is essential for the activation of cytotoxic T-cells and the destruction of **virus-infected or cancerous cells**. *HLA class III* - **HLA class III** genes encode for proteins involved in the immune system, such as factors of the **complement system** and cytokines like TNF-α. - They do not directly interact with **CD4** or **CD8** co-receptors for antigen presentation. *None of the options* - This option is incorrect because **CD4** is specifically associated with **HLA class II** in the context of antigen presentation. - The interaction between **CD4** and **MHC class II** is a fundamental principle of immunology.

Question 978: Which of the following components are involved in non-IgE mediated anaphylactic reactions?

A. Complement
B. Ig G
C. Ig M
D. All of the options (Correct Answer)

Explanation: ***All of the options*** - **Non-IgE mediated anaphylactic reactions** can involve various immune components beyond IgE, including **IgG**, **IgM**, and the **complement system**. - For instance, **IgG antibodies** can bind to mast cells or basophils and trigger degranulation, while **complement activation** can directly release anaphylatoxins, both leading to anaphylactoid symptoms. *Ig G* - While many anaphylactic reactions are **IgE-mediated**, **IgG antibodies** can also contribute to anaphylaxis, particularly in drug reactions or reactions to biologics. - **IgG-mediated anaphylaxis** often involves immune complexes that activate mast cells or basophils through Fcγ receptors. *Ig M* - **IgM antibodies** are less commonly implicated in direct anaphylactic reactions compared to IgE or IgG. - However, **IgM** can play a role in complex formation that activates the complement system, indirectly contributing to **anaphylactoid responses**. *Complement* - The **complement system** can be directly activated by certain drugs, physical stimuli, or immune complexes without the involvement of immunoglobulins. - This activation releases **anaphylatoxins (C3a, C4a, C5a)**, which can directly degranulate mast cells and basophils, leading to symptoms mimicking true anaphylaxis.

Question 979: Which enzyme secreted by neutrophils catalyzes the production of hypochlorous acid for pathogen killing?

A. An enzyme that catalyzes the production of hypochlorous acid (Correct Answer)
B. A protein involved in immune response
C. An enzyme that breaks down proteins
D. A type of carbohydrate

Explanation: ***An enzyme that catalyzes the production of hypochlorous acid*** - The enzyme secreted by neutrophils is **myeloperoxidase (MPO)**. - **MPO** catalyzes the reaction between **hydrogen peroxide (H₂O₂)** and **chloride ions (Cl⁻)** to produce **hypochlorous acid (HOCl)**, a highly potent antimicrobial agent. - This is part of the **oxygen-dependent (respiratory burst)** killing mechanism in neutrophils. - HOCl is one of the most effective microbicidal compounds, capable of destroying bacterial cell walls, viral envelopes, and fungal membranes. *A protein involved in immune response* - While MPO is indeed a protein involved in the immune response, this description is too **vague and non-specific**. - Many proteins participate in immune responses, but this doesn't capture MPO's specific **catalytic antimicrobial function**. - The question asks for the specific enzyme with a defined mechanism. *An enzyme that breaks down proteins* - This describes **proteases** such as **elastase, cathepsin G, and proteinase 3**, which are also present in neutrophil granules. - Proteases degrade proteins in pathogen cell walls and host tissue, but they are part of the **oxygen-independent** killing pathway. - The key enzyme for **hypochlorous acid production** is MPO, not proteases. *A type of carbohydrate* - Carbohydrates serve primarily as **energy sources and structural components**, not as antimicrobial enzymes. - The antimicrobial arsenal of neutrophils consists of **enzymes, reactive oxygen species (ROS), and antimicrobial peptides**, not carbohydrates. - This option is incorrect.

Question 980: What is the primary function of MHC Class I molecules?

A. Presentation of viral peptides to cytotoxic T cells (Correct Answer)
B. Facilitating graft rejection
C. Regulating immune responses
D. Activation of B cells

Explanation: ***Presentation of viral peptides to cytotoxic T cells*** - MHC Class I molecules primarily present **endogenous antigens** (e.g., viral or tumor peptides) to **CD8+ cytotoxic T lymphocytes (CTLs)**. - This presentation signals to the CTLs that the cell is infected or abnormal, leading to its destruction. *Facilitating graft rejection* - While MHC molecules (both Class I and Class II) are critical in graft rejection, their primary physiological function is not to facilitate rejection but rather to present antigens for immune surveillance. - Graft rejection is an undesirable consequence of the immune system's recognition of foreign MHC molecules on transplanted tissue. *Regulating immune responses* - MHC molecules do regulate immune responses by dictating which T cells can be activated, but this is a broader function mediated by various immune components. - Their specific and primary role is antigen presentation, which then leads to the regulation of T-cell responses. *Activation of B cells* - B cell activation primarily occurs through recognition of **native antigens** via their B cell receptor, often with help from **CD4+ T helper cells** that recognize antigens presented by **MHC Class II molecules**. - MHC Class I molecules are not directly involved in the activation of B cells.

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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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