Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 11: Which statement is true about immunoglobulins?

A. IgE has the maximum serum concentration.
B. IgG has the maximum serum concentration. (Correct Answer)
C. IgA has the minimum serum concentration.
D. IgM has the minimum serum concentration.

Explanation: **Explanation:** Immunoglobulins (antibodies) are glycoprotein molecules produced by plasma cells. Their distribution in human serum follows a specific hierarchy based on their biological roles and half-lives. **1. Why Option B is Correct:** **IgG** is the most abundant class of immunoglobulin in the serum, accounting for approximately **75–80%** of the total serum pool. This high concentration is due to its long half-life (about 23 days for IgG1, 2, and 4) and its role as the primary mediator of the secondary immune response. It is the only antibody capable of crossing the placenta, providing passive immunity to the fetus. **2. Why Other Options are Incorrect:** * **Option A:** **IgE** actually has the **lowest** serum concentration (approximately 0.002%). It is primarily involved in Type I hypersensitivity reactions and defense against helminthic infections. * **Options C & D:** **IgA** is the second most abundant (10–15%), found predominantly in seromucous secretions (colostrum, saliva, tears). **IgM** (5–10%) is the largest (pentamer) and the first to appear in a primary immune response. Neither has the minimum concentration; that distinction belongs to IgE. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Concentration:** **GAMED** (Ig**G** > Ig**A** > Ig**M** > Ig**D** > Ig**E**). * **IgG:** Only one to cross the placenta; fixes complement via the classical pathway. * **IgM:** Best at agglutination and complement fixation; indicates acute infection. * **IgA:** Secretory component protects it from enzymatic degradation in mucosal tracts. * **IgD:** Primarily acts as a B-cell antigen receptor.

Question 12: Antibodies to which of the following infections are not transmitted to a child?

A. Measles
B. Pertussis (Correct Answer)
C. Diphtheria
D. Polio

Explanation: **Explanation:** The transmission of maternal antibodies to the fetus occurs primarily via the placenta (IgG) and provides passive immunity during the first few months of life. However, the efficiency of this transfer and the resulting protection vary significantly depending on the specific pathogen. **Why Pertussis is the correct answer:** While maternal IgG antibodies against *Bordetella pertussis* are technically transferred across the placenta, they are **not protective**. The titers transferred are typically low and decay rapidly, leaving the newborn highly susceptible to "whooping cough" immediately after birth. This is why the **Tdap vaccine** is specifically recommended for pregnant women during the third trimester—to boost maternal antibody levels high enough to offer temporary protection until the infant can receive their own DTaP series at 6 weeks. **Analysis of Incorrect Options:** * **Measles:** Maternal antibodies (IgG) are highly effective and provide robust protection for the first 6–9 months of life. This is why the Measles vaccine (MR/MMR) is delayed until 9 months of age, as maternal antibodies would neutralize the live vaccine virus if given earlier. * **Diphtheria:** Protective levels of antitoxin are generally transferred to the fetus, provided the mother has been immunized or has natural immunity. * **Polio:** Maternal IgG provides significant systemic protection against paralytic poliomyelitis in the neonate for several months. **High-Yield Clinical Pearls for NEET-PG:** * **IgG** is the only immunoglobulin class that crosses the placenta (via neonatal Fc receptors - FcRn). * **IgA** is the primary immunoglobulin transferred via breast milk (colostrum), providing local mucosal immunity. * **Vaccination in Pregnancy:** Tdap is the "gold standard" for preventing neonatal pertussis. Live vaccines (like MMR and Varicella) are generally contraindicated during pregnancy.

Question 13: IFN-a and IFN-b are produced by the virus-infected cell due to the interaction of virus nucleic acid with which of the following?

A. C3 (third component of complement)
B. Defensins
C. TLR pathway (Correct Answer)
D. IL-12

Explanation: **Explanation:** **Why the Correct Answer is Right:** Interferon-alpha (IFN-α) and Interferon-beta (IFN-β) are **Type I Interferons**, which serve as the first line of defense against viral infections. When a virus infects a cell, its nucleic acids (dsRNA, ssRNA, or DNA) act as **PAMPs** (Pathogen-Associated Molecular Patterns). These are recognized by **Pattern Recognition Receptors (PRRs)**, most notably the **Toll-Like Receptors (TLRs)**. * **TLR3** recognizes dsRNA. * **TLR7/8** recognizes ssRNA. * **TLR9** recognizes unmethylated CpG DNA. Activation of these TLR pathways triggers signaling cascades (via IRF3/IRF7) that lead to the transcription and secretion of Type I IFNs, which induce an "antiviral state" in neighboring cells. **Why Incorrect Options are Wrong:** * **A. C3 (Complement):** C3 is central to the complement cascade (opsonization and MAC formation) but does not directly sense intracellular viral nucleic acids to induce interferon production. * **B. Defensins:** These are antimicrobial peptides (AMPs) that act primarily by disrupting the cell membranes of bacteria and fungi; they are not the primary triggers for the interferon response. * **D. IL-12:** This is a cytokine produced by macrophages and dendritic cells that stimulates NK cells and T-cells to produce **IFN-gamma (Type II Interferon)**, not Type I (α/β). **High-Yield Clinical Pearls for NEET-PG:** * **Type I IFNs (α, β):** Produced by almost all virus-infected cells (especially Plasmacytoid Dendritic Cells). * **Type II IFN (γ):** Produced by NK cells and Th1 cells; primary role is macrophage activation. * **Mechanism of Action:** IFNs induce the production of **Protein Kinase R (PKR)**, which inhibits protein synthesis, and **RNase L**, which degrades viral RNA, effectively halting viral replication. * **Therapeutic Use:** IFN-α is used in the treatment of Hepatitis B, Hepatitis C, and Kaposi Sarcoma.

Question 14: Where does B cell maturation take place?

A. Thymus
B. Lymph node
C. Bone marrow (Correct Answer)
D. Spleen

Explanation: **Explanation:** The maturation of B cells is a critical process in the development of the adaptive immune system. **1. Why Bone Marrow is correct:** In humans, the **Bone Marrow** serves as the primary lymphoid organ for B cell development. This process involves the rearrangement of immunoglobulin genes (V-D-J recombination) to produce a functional B-cell receptor (BCR). It is here that B cells undergo **negative selection** to ensure central tolerance, where self-reactive B cells are eliminated or undergo receptor editing. The "B" in B cell historically refers to the *Bursa of Fabricius* in birds, but in humans, it conveniently corresponds to **B**one marrow. **2. Why other options are incorrect:** * **Thymus:** This is the primary lymphoid organ for **T cell** maturation. Progenitor cells migrate from the bone marrow to the thymus to undergo TCR rearrangement and selection. * **Lymph Node & Spleen:** These are **secondary lymphoid organs**. They are sites where mature, naive B cells encounter antigens, undergo activation, class switching, and somatic hypermutation. They are sites of *proliferation* and *differentiation*, not initial maturation. **Clinical Pearls for NEET-PG:** * **Antigen-Independent Phase:** B cell maturation in the bone marrow is antigen-independent. * **Antigen-Dependent Phase:** Activation in the lymph nodes/spleen is antigen-dependent. * **Bruton’s Agammaglobulinemia:** A high-yield clinical correlation where B cell maturation is arrested at the **Pre-B cell stage** due to a deficiency in Bruton Tyrosine Kinase (BTK), leading to a lack of mature B cells and antibodies. * **Marker of Mature B cells:** The presence of both **surface IgM and IgD** signifies a mature, naive B cell ready to leave the bone marrow.

Question 15: Which of the following statements is true regarding the hybridoma technique?

A. It is a method for the production of monoclonal antibodies.
B. B cells are fused with myeloma cells using polyethylene glycol.
C. HAT medium is used to differentiate fused from unfused cells.
D. All of the above. (Correct Answer)

Explanation: The **Hybridoma Technique**, pioneered by Kohler and Milstein, is a cornerstone of modern immunology used to produce **monoclonal antibodies (mAbs)**—identical antibodies derived from a single clone of B cells. ### **Detailed Explanation** * **Option A:** The primary goal of this technique is to create a "hybrid" cell that possesses the **specificity** of a B cell (producing a single type of antibody) and the **immortality** of a myeloma cell. * **Option B:** To achieve fusion, splenic B cells (from an immunized animal) are mixed with cancerous plasma cells (myeloma cells). **Polyethylene Glycol (PEG)** acts as a fusogen, destabilizing cell membranes to allow the two different cell types to merge into a single hybridoma. * **Option C:** **HAT (Hypoxanthine, Aminopterin, and Thymidine) medium** is the selective agent. * **Aminopterin** blocks the *de novo* pathway of DNA synthesis. * **Unfused myeloma cells** die because they lack the HGPRT enzyme (salvage pathway). * **Unfused B cells** die naturally due to their short lifespan. * Only **hybridomas** survive because they inherit the salvage pathway from the B cell and immortality from the myeloma cell. Since all statements accurately describe the process, **Option D** is the correct answer. ### **High-Yield Clinical Pearls for NEET-PG** * **HGPRT Enzyme:** The myeloma cells used must be HGPRT-deficient. * **Applications:** Monoclonal antibodies are used in **ELISA**, **Flow Cytometry**, and therapeutic agents like **Rituximab** (anti-CD20) or **Infliximab** (anti-TNFα). * **Nobel Prize:** Kohler and Milstein received the Nobel Prize in 1984 for this discovery. * **Humanization:** Most therapeutic mAbs are "humanized" (ending in *-zumab*) or "chimeric" (*-ximab*) to prevent human anti-mouse antibody (HAMA) reactions.

Question 16: Which of the following cells is primarily concerned with humoral immunity?

A. B cell (Correct Answer)
B. T cell
C. Basophil
D. Monocytes

Explanation: **Explanation:** The immune system is divided into two main arms: **Humoral Immunity** and **Cell-Mediated Immunity (CMI)**. **1. Why B cells are correct:** Humoral immunity is mediated by macromolecules found in extracellular fluids, primarily **antibodies** (immunoglobulins). **B cells** are the central players in this process. Upon encountering a specific antigen and receiving signals from T-helper cells, B cells differentiate into **plasma cells**, which secrete large quantities of antibodies. These antibodies neutralize toxins, opsonize pathogens, and activate the complement system. **2. Why other options are incorrect:** * **T cells:** These are the primary mediators of **Cell-Mediated Immunity**. CD8+ T cells (Cytotoxic) directly kill infected cells, while CD4+ T cells (Helper) coordinate the immune response. They do not produce antibodies. * **Basophils:** These are granulocytes involved in **Type I Hypersensitivity** reactions and defense against parasites. They release histamine and heparin but are not the primary cells of humoral immunity. * **Monocytes:** These are mononuclear phagocytes that circulate in the blood before migrating into tissues to become **macrophages**. They act as professional Antigen-Presenting Cells (APCs) and phagocytes, linking innate and adaptive immunity, but do not produce antibodies. **Clinical Pearls for NEET-PG:** * **B-cell Markers:** CD19, CD20, CD21 (receptor for EBV), and CD22. * **Memory:** Both B and T cells provide immunological memory, but humoral memory resides in **Memory B cells**. * **B-cell Maturation:** Occurs in the **Bone Marrow** (unlike T cells, which mature in the Thymus). * **Burkitt Lymphoma:** Associated with B-cell malignancy and the c-myc translocation t(8;14).

Question 17: Which of the following is NOT true regarding Natural Killer (NK) cells?

A. Active against malignant cells
B. Inactive MHC antigen for killing microorganisms (Correct Answer)
C. First line defense against viral infections
D. No prior sensitization required

Explanation: **Explanation:** Natural Killer (NK) cells are a subset of large granular lymphocytes that play a pivotal role in the innate immune system. Unlike T-cells, they do not require prior sensitization or MHC-restricted antigen presentation to function. **Why Option B is Correct:** NK cells do not require "inactive" MHC antigens to kill; rather, they operate on the **"Missing Self" hypothesis**. NK cells possess **Inhibitory Receptors** (e.g., KIR - Killer Immunoglobulin-like Receptors) that recognize **MHC Class I molecules** on healthy host cells. When these receptors bind to MHC-I, the killing signal is inhibited. If a cell lacks MHC-I (a common tactic used by viruses and tumors to evade T-cells), the inhibitory signal is lost, and the NK cell is activated to destroy the target. Therefore, MHC presence *prevents* killing, while its absence *triggers* it. **Analysis of Other Options:** * **Option A:** NK cells are essential for **immunosurveillance** and are highly effective at identifying and lysing malignant cells that have downregulated MHC-I. * **Option C:** They provide the **first line of defense** against viral infections, acting rapidly (within hours) before the adaptive T-cell response matures. * **Option D:** As part of the **innate immune system**, they lack antigen-specific receptors (like TCR or BCR) and do not require prior exposure to a pathogen to act. **High-Yield Clinical Pearls for NEET-PG:** * **Markers:** CD16 (FcγRIII, mediates Antibody-Dependent Cellular Cytotoxicity - ADCC) and CD56 (NCAM). * **Mechanism of Killing:** Release of **Perforins** (create pores) and **Granzymes** (induce apoptosis). * **Cytokine Activation:** Their activity is significantly enhanced by **IL-2, IL-12, and Interferon-α/β**. * **Origin:** They are derived from the Common Lymphoid Progenitor (CLP) but do not mature in the thymus.

Question 18: A 25-year-old man has a negative Dick test and a positive Schick test. These results indicate that he has:

A. Neutralizing antibodies against diphtheria
B. Neutralizing antibodies against scarlet fever (Correct Answer)
C. A defect in cellular immunity
D. Received the full complement of diphtheria-pertussis-tetanus (DPT) shots as a child

Explanation: ### Explanation This question tests your understanding of **in vivo neutralization tests** used to assess immunity against exotoxin-producing bacteria. #### 1. Why Option B is Correct * **Dick Test:** This test is used to determine susceptibility to **Scarlet Fever** (*Streptococcus pyogenes* erythrogenic toxin). * **Negative Result:** If the patient has pre-existing **neutralizing antibodies** (antitoxin), the injected toxin is neutralized, and no skin rash/erythema occurs. Therefore, a **negative Dick test indicates immunity** to scarlet fever. * **Schick Test:** This test determines susceptibility to **Diphtheria** (*Corynebacterium diphtheriae* toxin). * **Positive Result:** A positive test (erythema and edema at the injection site) indicates that the patient **lacks neutralizing antibodies**. Therefore, a **positive Schick test indicates susceptibility** to diphtheria. Combining these: The patient is immune to scarlet fever (Negative Dick) but susceptible to diphtheria (Positive Schick). #### 2. Why Other Options are Wrong * **Option A:** Incorrect because a **positive** Schick test means the patient **lacks** neutralizing antibodies against diphtheria. * **Option C:** Incorrect because these tests assess **humoral immunity** (antibody-mediated neutralization of toxins), not cellular immunity (which is tested by DTH tests like the Mantoux test). * **Option D:** Incorrect because if he had received the full DPT series and developed a proper immune response, his Schick test would be **negative**. #### 3. High-Yield Clinical Pearls for NEET-PG * **Schick Test:** Used to assess the status of Diphtheria immunity. A "Negative" result means the person is immune. * **Schultz-Charlton Reaction:** A diagnostic test for Scarlet Fever where antitoxin is injected into a rash; if the rash blanches (fades), it confirms the rash is due to scarlet fever. * **Dick Test:** Historically used to identify children susceptible to Scarlet Fever. * **Mnemonic:** **D**ick test for **D**ick (Scarlet) fever; **S**chick test for **S**trangle (Diphtheria).

Question 19: The exact part of an antigen that reacts with the immune system is called as?

A. Clone
B. Epitope (Correct Answer)
C. Idiotope
D. Effector

Explanation: ### Explanation **Correct Answer: B. Epitope** **1. Why Epitope is Correct:** An **epitope**, also known as an **antigenic determinant**, is the specific chemical group or molecular configuration on the surface of an antigen that is recognized by and binds to the immune system (specifically to antibodies, B-cells, or T-cells). Antigens are usually large molecules, but the immune response is directed only against these small, discrete sites. A single antigen can possess multiple different epitopes (multivalent), each capable of stimulating a distinct immune response. **2. Why Other Options are Incorrect:** * **A. Clone:** This refers to a population of genetically identical cells derived from a single parent cell (e.g., a clone of B-lymphocytes). It is a cellular unit, not a part of an antigen. * **C. Idiotope:** This is an individual determinant found on the **variable region** of an antibody or T-cell receptor. It is part of the "recognizer" (the antibody), not the "recognized" (the antigen). The sum of idiotopes on a single antibody is called an *Idiotype*. * **D. Effector:** In immunology, this refers to cells (like Plasma cells or Cytotoxic T-cells) or molecules (like complement) that carry out the actual elimination of the pathogen. **3. High-Yield Clinical Pearls for NEET-PG:** * **Paratope:** The specific part of the **antibody** that binds to the epitope. (Memory aid: **E**pitope is on the **E**nemy/Antigen). * **Haptens:** These are small molecules that are antigenic (can bind to antibodies/epitopes) but not immunogenic (cannot elicit an immune response) unless attached to a large **carrier protein**. * **T-cell vs. B-cell Epitopes:** B-cell epitopes can be linear or conformational (surface-exposed), while T-cell epitopes are always linear peptides presented by MHC molecules.

Question 20: What class of immunoglobulin is primarily responsible for the rapid increase in antibody levels during secondary exposure to an antigen?

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

Explanation: **Explanation:** The correct answer is **IgG**. This phenomenon is a hallmark of the **secondary (anamnestic) immune response**. Upon re-exposure to a specific antigen, memory B cells rapidly proliferate and differentiate into plasma cells. Unlike the primary response, which is slow and dominated by IgM, the secondary response is characterized by a shorter lag period, a higher magnitude of antibody production, and a prolonged duration. This is primarily due to **class switching**, where the immune system shifts to producing IgG, which has a higher affinity for the antigen. **Analysis of Options:** * **IgA (Option A):** Primarily involved in mucosal immunity and found in secretions (tears, saliva, colostrum). It does not drive the systemic secondary response. * **IgD (Option B):** Acts mainly as a surface receptor on B cells; its systemic function in secondary exposure is negligible. * **IgG (Option C):** The most abundant circulating antibody. It is the only immunoglobulin that crosses the placenta and is the "workhorse" of the secondary immune response. * **IgM (Option D):** The first antibody produced during a **primary** exposure. It has a pentameric structure and is an indicator of acute infection. **High-Yield Clinical Pearls for NEET-PG:** * **Primary vs. Secondary:** Primary response = IgM; Secondary response = IgG. * **Avidity vs. Affinity:** Secondary responses exhibit **Affinity Maturation**, meaning the IgG produced binds more tightly to the antigen over time. * **Placental Transfer:** IgG is the only antibody providing passive immunity to the fetus (IgG "Goes" to the fetus). * **Complement Activation:** IgM is the most efficient activator of the classical complement pathway, followed by IgG.

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Immunology — MCQs

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