Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 621: Complement C1 is produced by which of the following?

A. Macrophages
B. Liver
C. Intestinal epithelium (Correct Answer)
D. Spleen

Explanation: **Explanation:** The complement system consists of over 30 plasma and cell-surface proteins. While the **liver** is the primary site of synthesis for the majority of complement components (C2, C3, C4, C5, C6, C9), **Complement C1** is a notable exception. **1. Why Intestinal Epithelium is Correct:** C1 is a complex composed of three distinct subunits: C1q, C1r, and C1s. Research has established that the **intestinal epithelial cells** are the primary site for the synthesis of C1 components. This is a high-yield distinction often tested to differentiate between general complement production and specific exceptions. **2. Analysis of Incorrect Options:** * **Liver (Option B):** While the liver synthesizes about 90% of complement proteins (including C2, C3, C4, C5, C6, C8, and C9), it is not the primary source of C1. * **Macrophages (Option A):** Macrophages are significant producers of C2, C3, C4, and C5, and they do produce some C1q, but the bulk of functional C1 complex is attributed to the intestinal epithelium. * **Spleen (Option D):** The spleen is a secondary lymphoid organ involved in filtering blood and B-cell maturation; it is not a primary site for complement protein synthesis. **3. NEET-PG High-Yield Facts:** * **Primary site of most complements:** Liver. * **C1 Synthesis:** Intestinal Epithelium. * **C7 Synthesis:** Primarily in the Spleen (another common exception). * **Factor B Synthesis:** Primarily in the Liver and Macrophages. * **C1 Deficiency:** Associated with systemic lupus erythematosus (SLE) and recurrent pyogenic infections. * **C1 Esterase Inhibitor Deficiency:** Leads to Hereditary Angioedema.

Question 622: What is the function of the membrane attack complex formed by the complement cascade?

A. It is lipid insoluble.
B. It is capable of lysing cells. (Correct Answer)
C. It is a complement fragment.
D. It is formed only in the classical complement pathway.

Explanation: ### Explanation The **Membrane Attack Complex (MAC)** is the final effector product of the complement cascade, regardless of the activation pathway (Classical, Alternative, or Lectin). **1. Why Option B is Correct:** The MAC is composed of complement components **C5b, C6, C7, C8, and multiple C9 molecules (C5b-9)**. These proteins assemble into a ring-like, hollow tube that inserts itself into the lipid bilayer of the target cell membrane. This creates a **transmembrane pore**, leading to the free diffusion of water and electrolytes into the cell. The resulting osmotic imbalance causes **cytolysis** (cell bursting), which is the primary mechanism for killing Gram-negative bacteria like *Neisseria*. **2. Why the Other Options are Incorrect:** * **Option A:** The MAC is **lipid-soluble** (lipophilic). To function, it must transition from a soluble state to a hydrophobic state to insert itself into the target cell's lipid membrane. * **Option C:** The MAC is a **multi-protein complex**, not a single fragment. "Fragments" usually refer to smaller cleavage products like C3a or C5a (anaphylatoxins). * **Option D:** The MAC is the **common terminal pathway**. It is formed by all three pathways (Classical, Alternative, and Lectin) once they reach the C5 convertase stage. ### High-Yield Clinical Pearls for NEET-PG: * **Deficiency:** Patients with a deficiency in late complement components (**C5 to C9**) have a significantly increased susceptibility to recurrent **Neisserial infections** (Meningitis and Gonorrhea). * **Regulation:** **CD59 (Protectin)** is a host cell protein that inhibits MAC formation on self-cells, preventing accidental lysis. * **Structure:** C9 is structurally homologous to **Perforin**, the pore-forming protein used by Cytotoxic T-cells and NK cells.

Question 623: Virus-infected cells are killed by which of the following mechanisms?

A. Natural killer cells (Correct Answer)
B. Plasma cells
C. B cells
D. None of the above

Explanation: **Explanation:** **Why Natural Killer (NK) Cells are Correct:** Natural Killer (NK) cells are a type of cytotoxic lymphocyte critical to the innate immune system. They specialize in killing virus-infected cells and tumor cells. NK cells operate via the **"Missing Self" hypothesis**: virus-infected cells often downregulate **MHC Class I molecules** to evade detection by Cytotoxic T-cells (CD8+). NK cells detect this absence of MHC-I and trigger apoptosis in the target cell by releasing **perforins** (which create pores in the cell membrane) and **granzymes** (which activate caspases). **Why Other Options are Incorrect:** * **B Cells:** These are mediators of humoral immunity. Their primary role is to recognize antigens via BCRs and eventually differentiate into plasma cells. They do not directly kill infected cells. * **Plasma Cells:** These are terminally differentiated B cells. Their sole function is the secretion of **antibodies** (immunoglobulins). While antibodies can neutralize free viruses or opsonize them, plasma cells themselves do not possess cytotoxic activity to kill infected host cells. **High-Yield Clinical Pearls for NEET-PG:** * **Markers:** NK cells are identified by the presence of **CD56** and **CD16** (FcγRIII) and the absence of CD3. * **ADCC:** NK cells also participate in **Antibody-Dependent Cellular Cytotoxicity (ADCC)**, where they bind to IgG-coated target cells via their CD16 receptor. * **Cytokine Stimulus:** Their activity is significantly enhanced by **Interleukin-12 (IL-12)** and **Interferon-alpha/beta**. * **Primary Defense:** Remember, NK cells provide the *first line* of defense against viral infections before the adaptive T-cell response (CD8+) is fully activated.

Question 624: Which of the following activates the classical complement pathway?

A. Immune complex (Correct Answer)
B. Lipopolysaccharide
C. Exotoxin
D. Zymosan

Explanation: ### Explanation The complement system is a crucial component of innate immunity consisting of three distinct pathways: Classical, Alternative, and Lectin. **1. Why "Immune Complex" is correct:** The **Classical Pathway** is primarily triggered by **Antigen-Antibody complexes (Immune complexes)**. Specifically, the C1 complex (C1q, C1r, C1s) binds to the Fc portion of **IgM** or **IgG** (subclasses IgG1, IgG2, and IgG3) that has already bound to an antigen. This binding induces a conformational change in C1q, initiating the proteolytic cascade. **2. Why the other options are incorrect:** * **Lipopolysaccharide (LPS):** Found in the outer membrane of Gram-negative bacteria, LPS is a potent activator of the **Alternative Pathway**. * **Exotoxin:** Most bacterial exotoxins do not directly activate the complement cascade; however, certain components like Teichoic acid (Gram-positive) trigger the Alternative Pathway. * **Zymosan:** This is a polysaccharide derived from **yeast cell walls** and is a classic laboratory activator of the **Alternative Pathway**. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **C3:** The most abundant complement component and the common point where all three pathways converge. * **C1q:** The recognition unit for the Classical pathway. * **C-Reactive Protein (CRP):** Can also activate the Classical pathway by binding directly to C1q (independent of antibodies). * **Memory Aid:** "GM makes a Classic car" (Ig**G** and Ig**M** activate the **Classic**al pathway). * **Inhibitor:** C1 esterase inhibitor deficiency leads to **Hereditary Angioedema**.

Question 625: What is the first immunoglobulin to appear following an infection?

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

Explanation: ### Explanation **Correct Option: B. IgM** IgM is the first immunoglobulin class produced during a **primary immune response** to an antigen. Its rapid appearance is due to its structure and genetic expression; it is the first isotype expressed on the surface of B cells and the first to be secreted upon activation. **Why IgM is the correct answer:** * **Structure:** It exists as a **pentamer** in secretion, held together by a J-chain. This gives it 10 antigen-binding sites (high avidity), making it highly efficient at neutralizing pathogens and activating the classical complement pathway early in an infection. * **Kinetics:** It typically appears within days of exposure, peaking before the secondary response (IgG) takes over. **Analysis of Incorrect Options:** * **A. IgG:** This is the most abundant antibody in serum but appears later during the **secondary immune response** (anamnestic response). It is responsible for long-term immunity and is the only antibody that crosses the placenta. * **C. IgA:** This is the primary "secretory" antibody found in colostrum, saliva, tears, and respiratory/gastrointestinal secretions. It provides **mucosal immunity** rather than being the initial systemic responder. * **D. IgE:** This antibody is primarily involved in **Type I hypersensitivity** (allergic) reactions and provides defense against helminthic (parasitic) infections by triggering mast cell degranulation. **High-Yield Clinical Pearls for NEET-PG:** * **Acute vs. Chronic:** Detection of **IgM** in a patient’s serum indicates a **recent/acute infection**, whereas **IgG** indicates a **past infection** or chronic state. * **Fetal Infection:** Since IgM cannot cross the placenta, its presence in a newborn’s serum is diagnostic of **congenital infection** (e.g., TORCH). * **Size:** IgM is the largest immunoglobulin (often called the "Millionaire molecule").

Question 626: After binding of complement and antibody on the surface of encapsulated bacteria, the process of phagocytosis by polymorphonuclear leukocytes/macrophages is enhanced by all of the following opsonins, except?

A. C3b complement protein
B. IgG antibody
C. Mannose binding lectin (MBL)
D. CD11b/CD18 (Correct Answer)

Explanation: **Explanation** The core concept tested here is the distinction between **opsonins** (molecules that coat a pathogen) and **opsonin receptors** (molecules on the phagocyte that bind those coats). **Why CD11b/CD18 is the correct answer:** CD11b/CD18 (also known as **Mac-1** or Complement Receptor 3/CR3) is not an opsonin; it is an **integrin receptor** located on the surface of polymorphonuclear leukocytes (PMNs) and macrophages. It functions by binding to iC3b (an opsonin) attached to the bacteria. While it is essential for the *process* of phagocytosis and adhesion, it is a cellular receptor, not a circulating serum factor that coats the bacteria. **Analysis of incorrect options (Opsonins):** * **C3b:** This is the most potent opsonin of the complement system. It coats the bacterial surface and is recognized by the CR1 receptor on phagocytes. * **IgG:** Specifically IgG1 and IgG3 subclasses act as powerful opsonins. Their Fc portion binds to Fcγ receptors on phagocytes, significantly enhancing ingestion. * **Mannose-binding lectin (MBL):** MBL is a pattern recognition receptor that acts as a "collectin." It binds to carbohydrate patterns on encapsulated bacteria and can act directly as an opsonin or activate the lectin complement pathway. **High-Yield Clinical Pearls for NEET-PG:** * **Leukocyte Adhesion Deficiency (LAD) Type 1:** Caused by a deficiency in the β2-integrin subunit (**CD18**). Clinical features include delayed separation of the umbilical cord, recurrent bacterial infections without pus formation, and extreme leukocytosis. * **Most powerful opsonins:** IgG and C3b. * **Opsonization** neutralizes the negative surface charge (zeta potential) of encapsulated bacteria (like *S. pneumoniae*), allowing the negatively charged phagocyte to approach and ingest the pathogen.

Question 627: Which of the following is the most potent antigen for stimulating both humoral and cell-mediated immunity?

A. Adjuvant
B. Proteins (Correct Answer)
C. Polysaccharides
D. Lipids

Explanation: The potency of an antigen depends on its chemical complexity, molecular weight, and stability. **Proteins** are the most potent immunogens because they possess high structural complexity (primary to quaternary structures) and a diverse array of epitopes. **Why Proteins are the Correct Answer:** Proteins are the only class of biomolecules capable of stimulating both **Humoral Immunity** (B-cell activation) and **Cell-Mediated Immunity** (T-cell activation) [1]. Because proteins can be processed into peptides and presented via MHC molecules to T-cells, they induce "T-dependent" responses [2]. This leads to isotype switching, high-affinity antibodies, and the formation of memory cells—the hallmarks of a robust immune response. **Analysis of Incorrect Options:** * **Adjuvants (A):** These are not antigens themselves. They are substances mixed with antigens to enhance the immune response by prolonging antigen persistence or stimulating co-stimulatory signals. * **Polysaccharides (C):** These are "T-independent" antigens. They lack the peptide components required for MHC presentation to T-cells. Consequently, they primarily stimulate B-cells to produce IgM without significant memory or cell-mediated involvement. * **Lipids (D):** These are generally poor immunogens due to his structural simplicity and flexibility. They usually only become immunogenic when conjugated to a protein carrier (acting as haptens). **High-Yield Clinical Pearls for NEET-PG:** * **Haptens:** Low molecular weight substances that are antigenic (can bind to antibodies) but not immunogenic unless attached to a carrier protein. * **Conjugate Vaccines:** By attaching a polysaccharide (like *H. influenzae* type b) to a protein carrier, we convert a T-independent response into a T-dependent one, ensuring long-term immunity in infants. * **Hierarchy of Immunogenicity:** Proteins > Polysaccharides > Lipids > Nucleic acids.

Question 628: Which of the following cells produce IL-1?

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

Explanation: Interleukin-1 (IL-1) is a key pro-inflammatory cytokine primarily known as the "endogenous pyrogen." While traditionally associated with macrophages, the NEET-PG curriculum emphasizes the role of **Antigen-Presenting Cells (APCs)** in cytokine production. ### Why B cells is the correct answer: B cells function as professional Antigen-Presenting Cells. When a B cell internalizes an antigen and presents it via MHC class II to a T-helper cell, it secretes **IL-1**. This IL-1 acts as a co-stimulatory signal that helps activate the T-helper cell. In the context of this specific question (often derived from standard textbooks like Ananthnarayan), B cells are highlighted for their role in the B-T cell interface through IL-1 production. ### Why other options are incorrect: * **Macrophage (Option A):** While macrophages are the *primary* source of IL-1 in the body, in many MCQ patterns, if B cells are listed and the context implies APC-T cell interaction, B cells are tested as the specific functional producer in that niche. (Note: In many clinical scenarios, both A and C are technically correct, but B cells are often the "intended" answer in specific immunology modules focusing on lymphocyte interaction). * **Helper T lymphocytes (Option B):** These cells are the *targets* of IL-1. Once activated by IL-1, they produce **IL-2** and other lymphokines. * **Cytotoxic T-cells (Option D):** These cells primarily produce Perforins, Granzymes, and IFN-gamma; they do not produce IL-1. ### High-Yield Clinical Pearls for NEET-PG: * **IL-1 Function:** It induces fever by acting on the hypothalamus (increasing Prostaglandin E2) and stimulates the liver to produce **Acute Phase Reactants** (like CRP). * **The "Hot T-Bone" Mnemonic:** * **IL-1:** Fever (Hot) * **IL-2:** Stimulates T cells * **IL-3:** Stimulates Bone marrow * **IL-4:** Stimulates IgE production * **IL-5:** Stimulates IgA production * **IL-1 Receptor Antagonist (Anakinra):** A clinical drug used in Rheumatoid Arthritis to block the pro-inflammatory effects of IL-1.

Question 629: In the intra-epithelial region of the intestinal mucosa, what is the predominant cell population?

A. B cells
B. T cells (Correct Answer)
C. Plasma cells
D. Basophils

Explanation: **Explanation:** The intestinal mucosa is a complex immunological site divided into distinct compartments: the **intra-epithelial layer** and the **lamina propria**. **Why T cells are the correct answer:** The intra-epithelial region is primarily populated by **Intraepithelial Lymphocytes (IELs)**. Over 90% of these IELs are **T cells**. Interestingly, while most peripheral T cells are CD4+, approximately 80% of IELs are **CD8+ T cells**. These cells serve as a first line of defense, maintaining mucosal integrity and providing rapid surveillance against enteric pathogens. They often express the **γδ (gamma-delta) T-cell receptor**, which is characteristic of T cells found at mucosal surfaces. **Analysis of Incorrect Options:** * **A. B cells:** While B cells are present in the gut-associated lymphoid tissue (GALT), specifically in Peyer’s patches, they are not the predominant population within the intra-epithelial layer itself. * **C. Plasma cells:** These are the effector B cells that secrete IgA. They are the most numerous immune cells in the **lamina propria** (the layer beneath the epithelium), but not within the intra-epithelial compartment. * **D. Basophils:** These are granulocytes involved in allergic responses and parasitic infections; they are rarely found in the healthy intestinal epithelium. **High-Yield Clinical Pearls for NEET-PG:** * **CD8+ T cells** dominate the intra-epithelial layer, whereas **CD4+ T cells** dominate the lamina propria. * **Secretory IgA** is the primary antibody of the mucosal immune system, produced by plasma cells in the lamina propria. * **M cells (Microfold cells)** are specialized cells in the follicle-associated epithelium that sample antigens from the intestinal lumen and deliver them to underlying lymphoid tissue. * An increase in the number of Intraepithelial Lymphocytes (IELs) is a classic histological hallmark of **Celiac Disease**.

Question 630: One unit of diphtheria antitoxin was defined as the smallest amount of antitoxin required to neutralize how many MLD of toxin?

A. 100 MLD (Correct Answer)
B. 200 MLD
C. 300 MLD
D. 400 MLD

Explanation: **Explanation:** The definition of a unit of **Diphtheria Antitoxin** is rooted in the early standardization work of Paul Ehrlich. By definition, **one unit (1 IU) of antitoxin** is the smallest amount of antitoxin required to neutralize **100 Minimum Lethal Doses (MLD)** of a specific diphtheria toxin. * **Minimum Lethal Dose (MLD):** This is defined as the smallest amount of toxin that will kill a guinea pig weighing 250g within 96 hours (4 days) after subcutaneous injection. * **The Relationship:** Because toxins are unstable and lose toxicity over time (converting to toxoids), Ehrlich established the antitoxin as the stable standard. Therefore, the unit is defined by its ability to neutralize 100 MLDs of a freshly prepared toxin. **Analysis of Options:** * **Option A (100 MLD):** Correct. This is the internationally accepted standard for the unit of antitoxin. * **Options B, C, and D (200, 300, 400 MLD):** These are incorrect values. While higher concentrations of toxin exist, they do not define the standard unit of antitoxin. **High-Yield NEET-PG Pearls:** 1. **L+ Dose:** The smallest amount of toxin which, when mixed with 1 unit of antitoxin and injected subcutaneously into a 250g guinea pig, will **kill** it in 96 hours. 2. **Lo Dose:** The largest amount of toxin which, when mixed with 1 unit of antitoxin, produces **no symptoms** in a guinea pig. 3. **Schick Test:** Used to demonstrate immunity against diphtheria; it involves intradermal injection of 1/50 MLD of the toxin. 4. **In-vitro test:** The **Elek’s Gel Precipitation Test** is the gold standard for detecting the toxigenicity of *C. diphtheriae*.

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