Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 571: CD4 lymphocytes (helper cells) recognize which of the following antigens?

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

Explanation: ### Explanation The recognition of antigens by T-lymphocytes is governed by the principle of **MHC Restriction**. T-cells do not recognize free-floating antigens; they only respond to processed antigenic peptides presented on **Major Histocompatibility Complex (MHC)** molecules, also known as **Human Leukocyte Antigens (HLA)** in humans. **Why Option B is Correct:** CD4+ T-cells (Helper T-cells) are restricted to **HLA Class II** molecules. These antigens are primarily expressed on **Professional Antigen-Presenting Cells (APCs)** such as macrophages, B-cells, and dendritic cells. When an APC engulfs an exogenous pathogen, it presents the peptide via HLA Class II to the CD4+ cell, triggering the release of cytokines and the orchestration of the adaptive immune response. **Why Other Options are Incorrect:** * **Option A (HLA Class I):** These are recognized by **CD8+ T-cells** (Cytotoxic T-cells). HLA Class I molecules are found on almost all nucleated cells and present endogenous antigens (like viral or tumor proteins) to signal the destruction of the infected cell. * **Option C (HLA Class III):** These genes encode various components of the complement system (C2, C4), tumor necrosis factor (TNF), and heat shock proteins. They are **not** involved in the direct presentation of antigens to T-cells. **High-Yield NEET-PG Pearls:** 1. **The Rule of 8:** A simple mnemonic to remember MHC restriction: * **CD4 × MHC II = 8** * **CD8 × MHC I = 8** 2. **HLA Loci:** Class I includes HLA-A, B, and C. Class II includes HLA-DP, DQ, and DR. 3. **Exogenous vs. Endogenous:** CD4/MHC II deals with exogenous (extracellular) antigens, while CD8/MHC I deals with endogenous (intracellular) antigens.

Question 572: Which of the following aids in the killing of intracellular bacteria?

A. Histamine
B. Interleukin-2
C. Catalase
D. Lysozyme (Correct Answer)

Explanation: **Explanation:** **Correct Option: D. Lysozyme** Lysozyme (muramidase) is a primary component of the innate immune system found in mucosal secretions (tears, saliva, mucus) and within the primary granules of neutrophils and macrophages. It functions by cleaving the **β-1,4 glycosidic bond** between N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) in the bacterial **peptidoglycan** cell wall. This enzymatic degradation leads to osmotic lysis and is a critical mechanism for killing bacteria that have been internalized via phagocytosis. **Analysis of Incorrect Options:** * **A. Histamine:** Released primarily by mast cells and basophils, histamine is a vasoactive amine. It mediates vasodilation and increased vascular permeability during acute inflammation and Type I hypersensitivity, but it lacks direct bactericidal activity. * **B. Interleukin-2 (IL-2):** This is a cytokine produced by T-cells (Th1). Its primary role is the proliferation and differentiation of T-cells, B-cells, and Natural Killer (NK) cells. While it coordinates the immune response, it does not directly kill bacteria. * **C. Catalase:** This is a bacterial enzyme (virulence factor), not a host defense mechanism. Catalase-producing bacteria (e.g., *Staphylococcus aureus*) use it to neutralize hydrogen peroxide ($H_2O_2$) into water and oxygen, thereby evading the oxidative burst of phagocytes. **High-Yield Clinical Pearls for NEET-PG:** * **Lysozyme** is more effective against Gram-positive bacteria because their peptidoglycan layer is more accessible than in Gram-negative bacteria (which have an outer membrane). * **Chronic Granulomatous Disease (CGD):** A deficiency in NADPH oxidase leads to an inability to produce reactive oxygen species. Patients are specifically susceptible to **Catalase-positive** organisms. * **Defensins:** Along with lysozyme, these are small cationic proteins in phagocytes that create pores in bacterial membranes, aiding in intracellular killing.

Question 573: Which of the following is NOT a type II hypersensitivity reaction?

A. Stevens-Johnson syndrome (Correct Answer)
B. Drug-induced hemolytic anemia
C. Drug-induced thrombocytopenia
D. Hemolytic disease of the newborn

Explanation: **Explanation:** Hypersensitivity reactions are classified based on the immune mechanism involved. **Type II hypersensitivity** (Cytotoxic) involves IgG or IgM antibodies binding to antigens on specific cell surfaces or tissues, leading to cell destruction via the complement system or phagocytosis. **Why Stevens-Johnson Syndrome (SJS) is the correct answer:** SJS is a **Type IV (Delayed-type) hypersensitivity reaction**, specifically a T-cell mediated cytotoxic response. It involves the activation of CD8+ cytotoxic T-lymphocytes and Natural Killer cells, which release proteins like granulysin and perforin. This leads to widespread keratinocyte apoptosis and epidermal detachment. It is not mediated by antibodies, which is the hallmark of Type II. **Analysis of incorrect options (Type II Reactions):** * **Drug-induced hemolytic anemia:** Drugs (like penicillin) act as haptens, binding to RBC membranes. Antibodies then target these drug-coated cells, leading to hemolysis. * **Drug-induced thrombocytopenia:** Similar to hemolytic anemia, drugs bind to platelets, triggering antibody-mediated destruction. * **Hemolytic disease of the newborn (HDN):** Maternal IgG antibodies cross the placenta and attack fetal Rh+ red blood cells. **NEET-PG High-Yield Pearls:** * **Mnemonic for Hypersensitivity:** **ACID** (Type I: **A**llergic/Anaphylactic; Type II: **C**ytotoxic; Type III: **I**mmune-Complex; Type IV: **D**elayed). * **SJS vs. TEN:** They are differentiated by the percentage of body surface area (BSA) involved: SJS <10%, Overlap 10-30%, Toxic Epidermal Necrolysis (TEN) >30%. * **Common SJS Triggers:** Sulfonamides, Antiepileptics (Phenytoin, Carbamazepine), and Allopurinol.

Question 574: Innate immunity is stimulated by which part of bacteria that acts as a danger signal?

A. Cell wall carbohydrate sequence (Correct Answer)
B. Flagella
C. Bacterial cell membrane
D. Nucleus

Explanation: **Explanation:** Innate immunity relies on the recognition of conserved molecular structures known as **Pathogen-Associated Molecular Patterns (PAMPs)**. These are recognized by **Pattern Recognition Receptors (PRRs)**, such as Toll-like receptors (TLRs), on host immune cells. **Why Option A is correct:** The **cell wall carbohydrate sequences** (such as peptidoglycan, lipopolysaccharide/LPS, and mannose) are classic examples of PAMPs. These structures are essential for bacterial survival and are unique to microbes, allowing the innate immune system to identify them as "danger signals" or "non-self." For instance, LPS (found in Gram-negative cell walls) is a potent stimulator of TLR-4, triggering a robust inflammatory response. **Why the other options are incorrect:** * **B. Flagella:** While flagellin (the protein in flagella) is a PAMP recognized by TLR-5, the question specifically asks for the primary "danger signal" often associated with structural sequences. In the hierarchy of innate triggers, cell wall components are the most ubiquitous and potent initiators. * **C. Bacterial cell membrane:** While membranes contain phospholipids, they are structurally similar to host membranes. The innate system specifically targets the **cell wall** (which humans lack) rather than the generic lipid bilayer. * **D. Nucleus:** Bacteria are prokaryotes and **do not possess a nucleus**. Their genetic material is organized in a nucleoid. **High-Yield Clinical Pearls for NEET-PG:** * **TLR-4** recognizes LPS (Gram-negative bacteria). * **TLR-2** recognizes Peptidoglycan and Teichoic acid (Gram-positive bacteria). * **TLR-5** recognizes Flagellin. * **TLR-9** recognizes unmethylated CpG DNA. * **DAMPs (Damage-Associated Molecular Patterns):** These are endogenous danger signals released from *host* cells during injury (e.g., Heat shock proteins, Uric acid).

Question 575: Which are the terminal components of the complement system?

A. C6, C7, C8
B. C7, C8, C9 (Correct Answer)
C. C3, C4, C5
D. C5, C6, C7

Explanation: **Explanation:** The complement system consists of a cascade of proteins that mediate inflammation and pathogen destruction. The **Terminal Complement Complex (TCC)**, also known as the **Membrane Attack Complex (MAC)**, is the final effector stage of all three complement pathways (Classical, Alternative, and Lectin). The MAC is formed by the sequential assembly of five proteins: **C5b, C6, C7, C8, and C9**. * The process begins when C5 is cleaved into C5a and C5b. * C5b binds to C6 and C7 to form a complex that inserts into the target cell membrane. * C8 then binds, followed by the polymerization of multiple **C9** molecules, which form a transmembrane pore leading to osmotic lysis of the cell. * While C5b-C9 constitutes the entire MAC, **C7, C8, and C9** are specifically categorized as the terminal components that complete the pore structure. **Analysis of Options:** * **Option B (Correct):** C7, C8, and C9 are the final structural components that anchor and form the functional pore. * **Option A & D:** These include C6 and C5/C7. While C5b and C6 are part of the MAC initiation, they are considered "late-acting" but not the final terminal steps compared to C7-C9. * **Option C:** C3, C4, and C5 are "early" components involved in opsonization (C3b) and the formation of C3/C5 convertases. **NEET-PG Clinical Pearls:** 1. **Deficiency:** Patients with deficiencies in terminal components (C5-C9) have a high susceptibility to recurrent **Neisseria** infections (Meningitis and Gonorrhea). 2. **Inhibitor:** **CD59 (Protectin)** is a host cell protein that inhibits the assembly of the MAC to prevent self-destruction. 3. **Anaphylatoxins:** C3a, C4a, and **C5a** (the most potent) are byproducts that trigger mast cell degranulation.

Question 576: Which of the following statements about immunoglobulins is true?

A. IgE fixes complement
B. IgM fixes complement (Correct Answer)
C. IgG is found in minimum concentration
D. IgG is elevated in primary immune response

Explanation: **Explanation:** **Why IgM is the correct answer:** Complement fixation via the **Classical Pathway** is initiated by the binding of C1q to the Fc portion of an antibody. **IgM** is the most potent activator of the complement system. This is because IgM exists as a **pentamer** in serum; its multimeric structure provides multiple closely spaced Fc fragments, allowing a single IgM molecule to bind and activate C1q efficiently. **Analysis of Incorrect Options:** * **Option A (IgE):** IgE does not fix complement. Its primary role is in Type I Hypersensitivity (allergic reactions) and defense against helminthic parasites by binding to mast cells and basophils via high-affinity FcεRI receptors. * **Option C (IgG concentration):** This is incorrect. **IgG is the most abundant** immunoglobulin in serum (approx. 75–80%), providing long-term immunity. IgE is actually found in the minimum (lowest) concentration in serum. * **Option D (Primary response):** **IgM** is the first antibody to appear in a primary immune response. IgG is the predominant antibody in the **secondary (anamnestic) response** due to class switching and memory B-cell activation. **NEET-PG High-Yield Pearls:** * **Complement Fixation:** Only **IgM** and **IgG** (subclasses IgG3 > IgG1 > IgG2) fix complement via the classical pathway. IgG4 does not. * **Placental Transfer:** **IgG** is the only immunoglobulin that crosses the placenta (providing passive immunity to the fetus). * **Secretory Component:** **IgA** is the primary mediator of mucosal immunity and contains a "J chain" (like IgM) and a secretory piece. * **Heat Lability:** **IgE** is unique for being heat-labile (inactivated at 56°C for 30 minutes).

Question 577: Which Toll-like receptor is associated with viruses?

A. TLR1
B. TLR2
C. TLR3 (Correct Answer)
D. TLR4

Explanation: **Explanation:** Toll-like receptors (TLRs) are a class of Pattern Recognition Receptors (PRRs) that play a crucial role in the innate immune system by detecting Pathogen-Associated Molecular Patterns (PAMPs). **1. Why TLR3 is Correct:** TLR3 is specifically localized in the **endosomal membranes** (intracellularly) rather than the cell surface. Its primary ligand is **double-stranded RNA (dsRNA)**, which is a hallmark of viral replication. When TLR3 detects dsRNA, it triggers the production of Type I Interferons (IFN-α and IFN-β), which are essential for the antiviral response. **2. Analysis of Incorrect Options:** * **TLR1:** This receptor forms a heterodimer with TLR2 to recognize **triacyl lipopeptides** found primarily in bacteria and mycobacteria. * **TLR2:** Located on the cell surface, it recognizes **peptidoglycan**, lipoteichoic acid, and lipoproteins. It is the primary receptor for **Gram-positive bacteria**. * **TLR4:** This is the classic receptor for **Lipopolysaccharide (LPS)** found in the outer membrane of **Gram-negative bacteria**. It is also associated with the MD2 and CD14 complex. **3. NEET-PG High-Yield Pearls:** * **Viral TLRs:** Remember the "Viral Trio": **TLR3** (dsRNA), **TLR7/8** (ssRNA), and **TLR9** (unmethylated CpG DNA). * **Location:** TLRs 1, 2, 4, 5, and 6 are on the **plasma membrane** (detecting extracellular pathogens). TLRs 3, 7, 8, and 9 are **endosomal** (detecting intracellular/nucleic acid PAMPs). * **TLR5:** Specifically recognizes **Flagellin** (bacterial flagella). * **Clinical Correlation:** Deficiency of TLR3 is associated with an increased susceptibility to **Herpes Simplex Encephalitis (HSE)**.

Question 578: Which antibody mediates the primary immune response?

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

Explanation: **Explanation:** The correct answer is **IgM**. **Why IgM is the correct answer:** IgM is the first antibody isotype produced by B cells upon initial exposure to a new antigen (the **primary immune response**). Structurally, it exists as a **pentamer** (five units joined by a J-chain), giving it 10 antigen-binding sites. This high valency allows it to bind effectively to pathogens even when the individual binding affinity is low. Its presence in serum typically indicates an acute or recent infection. **Why the other options are incorrect:** * **IgE:** Primarily involved in **Type I hypersensitivity** (allergic) reactions and host defense against helminthic (parasitic) infections. It binds to mast cells and basophils. * **IgA:** The predominant antibody in **mucosal secretions** (tears, saliva, colostrum, GI tract). it provides local immunity at body surfaces. * **IgD:** Found mainly on the surface of naive B lymphocytes, where it functions as an antigen receptor. It has no major known role in serum or primary systemic response. **High-Yield Clinical Pearls for NEET-PG:** * **IgG** is the antibody of the **secondary immune response** (anamnestic response) and is the only antibody that **crosses the placenta**. * **IgM** is the most efficient antibody at **complement fixation** (via the classical pathway). * **Isotype Switching:** The process where a B cell changes from producing IgM to IgG, IgA, or IgE is stimulated by cytokines and occurs in the germinal centers of lymph nodes. * **Molecular Weight:** IgM is the largest immunoglobulin ("Macroglobulin"), making it restricted primarily to the intravascular compartment.

Question 579: Interferon beta is stimulated by which type of infection?

A. Bacterial infection
B. Viral infection (Correct Answer)
C. Fungal infection
D. Mycoplasma infection

Explanation: **Explanation:** Interferons (IFNs) are a group of signaling proteins (cytokines) released by host cells in response to the presence of several pathogens. **Interferon-beta (IFN-β)**, along with Interferon-alpha (IFN-α), belongs to the **Type I Interferon** family. **Why Viral Infection is Correct:** The primary stimulus for the production of Type I Interferons is a **viral infection**. When a virus infects a cell, viral components (specifically double-stranded RNA or DNA) are recognized by Pattern Recognition Receptors (PRRs) like Toll-like receptors (TLR-3, 7, 8, 9). This triggers the synthesis of IFN-β. Once released, IFN-β binds to neighboring uninfected cells, inducing an **"antiviral state"** by stimulating the production of enzymes (like PKR and RNase L) that inhibit viral replication and protein synthesis. **Why Other Options are Incorrect:** * **Bacterial, Fungal, and Mycoplasma infections:** While these pathogens trigger the immune system, they primarily stimulate the production of different cytokines (like IL-1, IL-6, TNF-α) and Type II Interferon (IFN-γ) from T-cells and NK cells. They do not typically induce the high-level production of IFN-β characteristic of the innate antiviral response. **High-Yield Clinical Pearls for NEET-PG:** * **Type I IFNs (α, β):** Produced by almost all nucleated cells (especially Plasmacytoid Dendritic Cells for IFN-α); primary role is **Antiviral**. * **Type II IFN (γ):** Produced by Th1 cells and NK cells; primary role is **Immunomodulatory** (Macrophage activation). * **Clinical Use:** Recombinant **Interferon-beta** is a standard disease-modifying therapy used in the treatment of **Multiple Sclerosis (MS)** to reduce the frequency of relapses. * **Mechanism:** IFNs do not kill viruses directly; they inhibit viral translation and degrade viral mRNA.

Question 580: The reaction between an antibody and a soluble antigen is demonstrated by which of the following?

A. Agglutination
B. Complement fixation
C. Precipitation (Correct Answer)
D. Hemagglutination

Explanation: **Explanation:** The fundamental principle governing antigen-antibody (Ag-Ab) reactions is the physical state of the antigen. **1. Why Precipitation is Correct:** Precipitation occurs when a **soluble antigen** reacts with its specific antibody (precipitin) in the presence of electrolytes at an optimal temperature and pH. This interaction leads to the formation of an insoluble lattice that settles as a visible precipitate. This reaction is most efficient when Ag and Ab are in the **Zone of Equivalence** (Lattice Hypothesis). **2. Analysis of Incorrect Options:** * **Agglutination (A):** This involves a reaction between an antibody and a **particulate (insoluble) antigen** (e.g., bacteria, RBCs). The result is visible clumping rather than a fine precipitate. * **Complement Fixation (B):** This is a complex serological test used to detect specific antibodies. It relies on the consumption of complement by an Ag-Ab complex, which is then visualized using an indicator system (sensitized sheep RBCs). It does not inherently define the reaction of a soluble antigen. * **Hemagglutination (D):** This is a specific type of agglutination where the particulate antigens are **Red Blood Cells**. It is used for blood grouping and detecting viruses like Influenza. **3. High-Yield Clinical Pearls for NEET-PG:** * **Prozone Phenomenon:** False-negative precipitation/agglutination due to **antibody excess**. * **Postzone Phenomenon:** False-negative result due to **antigen excess**. * **Examples of Precipitation:** VDRL test (Flocculation), Elek’s gel precipitation test (for Diphtheria toxin), and Immunodiffusion (Mancini or Ouchterlony methods). * **Key Difference:** Soluble Ag = Precipitation; Particulate Ag = Agglutination.

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Immunology — MCQs

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