Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 271: Which immunoglobulin crosses the placenta?

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

Explanation: **Explanation:** **IgG** is the only class of immunoglobulin capable of crossing the placenta. This occurs through a specialized active transport mechanism involving **neonatal Fc receptors (FcRn)** located on the syncytiotrophoblast of the placenta. This transfer provides the fetus and newborn with **passive immunity**, protecting them during the first few months of life while their own immune system matures. **Analysis of Incorrect Options:** * **IgA (Option A):** Primarily found in secretions (tears, saliva, colostrum) as a dimer. It does not cross the placenta but is the main antibody transferred via **breast milk**, providing local intestinal immunity to the infant. * **IgE (Option B):** Involved in Type I hypersensitivity and parasitic infections. It has a very low serum concentration and lacks the specific Fc region required for placental transport. * **IgM (Option D):** It is a large pentamer (the "millionaire molecule"). Due to its high molecular weight and lack of specific receptors, it cannot cross the placenta. **Clinical Note:** The presence of IgM in a neonate’s blood indicates an **intrauterine infection** (e.g., TORCH), as it signifies the fetus's own immune response. **High-Yield Clinical Pearls for NEET-PG:** * **Subclasses:** Only IgG1, IgG3, and IgG4 cross the placenta efficiently; IgG2 crosses poorly. * **Rh Incompatibility:** Because IgG crosses the placenta, anti-D antibodies (IgG) cause Hemolytic Disease of the Newborn (HDN). * **Half-life:** IgG has the longest half-life (approx. 23 days) among all immunoglobulins. * **Abundance:** IgG is the most abundant immunoglobulin in the serum (75-80%).

Question 272: Cell fusion is an innovative technique for preparing specific antibodies. Which of the following techniques is NOT used to induce cell fusion?

A. Adhering some inactivated viruses to the surface membrane
B. Adding polyethylene glycol
C. Applying a mild electric shock
D. Reducing the viscosity of the membrane (Correct Answer)

Explanation: ### **Explanation** The question refers to the **Hybridoma Technique**, used for the production of **Monoclonal Antibodies (mAbs)**. This process involves fusing a specific antibody-producing B-cell with an immortal myeloma cell to create a "hybridoma." **Why Option D is Correct:** **Reducing the viscosity of the membrane** is not a standard or effective method for inducing cell fusion. While membrane fluidity is a physiological property of lipid bilayers, simply reducing viscosity does not provide the necessary physical or chemical stimulus required to break and reform the plasma membranes of two distinct cells into a single unit. **Analysis of Incorrect Options:** * **Option A (Inactivated Viruses):** Certain viruses, most notably the **Sendai virus** (an RNA virus), possess fusion proteins on their envelope. When inactivated (so they cannot replicate), they can still bind to cell surfaces and facilitate the merging of two different cell membranes. * **Option B (Polyethylene Glycol - PEG):** This is the **most common chemical method** used in laboratories. PEG acts as a dehydrating agent that brings membranes into close proximity, destabilizing them and allowing them to fuse. * **Option C (Mild Electric Shock):** Known as **Electrofusion**, this method involves applying a pulsed electrical field to the cell suspension. This creates temporary pores in the membrane (electroporation), leading to cell fusion when cells are in contact. --- ### **High-Yield Clinical Pearls for NEET-PG** * **Father of Monoclonal Antibodies:** Georges Köhler and César Milstein (Nobel Prize winners). * **Selection Medium:** The **HAT Medium** (Hypoxanthine, Aminopterin, and Thymidine) is used to select only the hybridoma cells. * **Aminopterin’s Role:** It blocks the *de novo* pathway of nucleotide synthesis, forcing cells to use the **Salvage Pathway** (which myeloma cells lack, ensuring only hybrids survive). * **Clinical Use:** Monoclonal antibodies are used in diagnosis (ELISA, RIA) and therapy (e.g., Rituximab for Lymphoma, Infliximab for RA).

Question 273: Which of the following is a component of innate immunity?

A. T lymphocytes
B. Complement proteins
C. B lymphocytes
D. NK cells (Correct Answer)

Explanation: **Explanation:** Innate immunity is the body's first line of defense, providing a rapid, non-specific response to pathogens without requiring prior exposure. **Natural Killer (NK) cells** are a critical cellular component of innate immunity. Unlike T or B cells, NK cells do not possess antigen-specific receptors; instead, they identify and destroy virally infected or tumor cells by detecting the absence of MHC class I molecules (the "missing self" hypothesis). **Analysis of Options:** * **NK Cells (Correct):** These are large granular lymphocytes that function in the innate system. They provide immediate defense and bridge the gap while the adaptive immune response is being primed. * **T Lymphocytes (Incorrect):** These are the mediators of **Cell-Mediated Immunity (CMI)**, a branch of adaptive immunity. They require antigen presentation via MHC molecules and undergo clonal expansion. * **B Lymphocytes (Incorrect):** These are the mediators of **Humoral Immunity**, another branch of adaptive immunity. They differentiate into plasma cells to produce specific antibodies. * **Complement Proteins (Incorrect):** While complement proteins are indeed part of innate immunity, in the context of this specific question (often sourced from standard textbooks like Ananthanarayan), **NK cells** are frequently highlighted as the primary "cellular" innate component. *Note: If the question allows for multiple answers, Complement is also innate, but in single-best-answer formats, cellular components like NK cells or Phagocytes are often the intended focus.* **High-Yield NEET-PG Pearls:** * **Markers:** NK cells are identified by the presence of **CD56** and **CD16** and the absence of CD3. * **Mechanism:** They release **perforins and granzymes** to induce apoptosis in target cells. * **Cytokine Link:** **Interleukin-12 (IL-12)** and **Interferon-alpha/beta** are potent activators of NK cells. * **Innate Components:** Include physical barriers (skin), chemical barriers (gastric acid), cells (Neutrophils, Macrophages, NK cells), and soluble factors (Complement, CRP).

Question 274: Allergic reactions are a type of hypersensitivity. To which type of hypersensitivity reaction do common allergies like hay fever and asthma belong?

A. Type I (Correct Answer)
B. Type II
C. Type III
D. Type IV

Explanation: **Explanation:** **Correct Answer: A. Type I Hypersensitivity** Type I hypersensitivity, also known as **Immediate Hypersensitivity**, is mediated by **IgE antibodies**. When a predisposed individual is exposed to an allergen (like pollen or dander), IgE binds to the surface of **mast cells and basophils** via Fc receptors. Upon re-exposure, the allergen cross-links these IgE molecules, triggering degranulation and the release of vasoactive amines like **histamine**. This leads to rapid clinical manifestations such as vasodilation and bronchoconstriction, characteristic of hay fever, extrinsic asthma, and anaphylaxis. **Why the other options are incorrect:** * **Type II (Cytotoxic):** Mediated by **IgG or IgM** antibodies directed against antigens on specific cell surfaces or tissues (e.g., ABO incompatibility, Rh incompatibility, Myasthenia Gravis). * **Type III (Immune-Complex):** Caused by the deposition of **antigen-antibody complexes** in tissues, leading to complement activation and inflammation (e.g., SLE, Post-streptococcal glomerulonephritis, Arthus reaction). * **Type IV (Delayed-type):** This is **cell-mediated** (T-cells), not antibody-mediated. It takes 48–72 hours to manifest (e.g., Mantoux test, contact dermatitis, graft rejection). **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic (ACID):** **A**naphyalctic (I), **C**ytotoxic (II), **I**mmune-Complex (III), **D**elayed (IV). * **Key Cells:** Mast cells are the primary effectors in Type I; T-lymphocytes/Macrophages in Type IV. * **Atopy:** Refers to the genetic predisposition to develop Type I reactions. * **Coombs and Gell Classification:** This is the formal name for the four-type classification system of hypersensitivity.

Question 275: What is the distinct amino acid sequence at the antigen combining site called?

A. Idiotype (Correct Answer)
B. Allotype
C. Epitope
D. Paratope

Explanation: ### Explanation The correct answer is **Idiotype**. **1. Why Idiotype is correct:** An **Idiotype** refers to the unique set of antigenic determinants (idiotopes) located within the **Variable (V) regions** of an antibody or T-cell receptor. These are determined by the specific **amino acid sequence** at the antigen-binding site. Since each clone of B-cells produces antibodies with a unique V-region to fit a specific antigen, the idiotype serves as a signature for that specific clone. **2. Why other options are incorrect:** * **Allotype:** Refers to antigenic variations in the **Constant (C) region** of antibodies that differ between individuals of the same species due to allelic inheritance (e.g., Gm markers in IgG). * **Epitope:** Also known as the antigenic determinant, this is the specific part of the **antigen** molecule to which an antibody binds. * **Paratope:** This is the actual **physical pocket** or site on the antibody (formed by the Hypervariable regions) that binds to the epitope. While closely related to idiotype, the "sequence" itself that defines the specificity is the idiotype. **3. High-Yield Clinical Pearls for NEET-PG:** * **Isotype:** Determines the class/subclass of the antibody (IgG, IgM, etc.) based on heavy chain constant regions; same in all individuals of a species. * **Hypervariable Regions (HVRs):** Also called **Complementarity Determining Regions (CDRs)**. There are 3 CDRs in the light chain and 3 in the heavy chain. These form the idiotype. * **Monoclonal Gammopathy:** In conditions like Multiple Myeloma, all myeloma cells produce antibodies with the **same idiotype**, appearing as an 'M-band' on electrophoresis.

Question 276: Which of the following statements is FALSE regarding Natural Killer (NK) cells?

A. NK cell activity is augmented by interferons.
B. NK cells provide the initial response in viral infections.
C. NK cells are active against tumor cells.
D. NK cell activity is MHC restricted. (Correct Answer)

Explanation: **Explanation:** **Why Option D is the correct (False) statement:** Natural Killer (NK) cells are a component of the **innate immune system**. Unlike T-cells, which require the presentation of antigens via Major Histocompatibility Complex (MHC) molecules to recognize targets, NK cells are **MHC-unrestricted**. In fact, NK cells operate on the "Missing Self" hypothesis: they preferentially kill cells that have *downregulated* or lost their MHC Class I expression (a common evasion tactic used by viruses and tumors). **Analysis of other options:** * **Option A:** Interferons (IFN-α and IFN-β) and Interleukin-12 are potent activators of NK cells, enhancing their cytotoxic capabilities during an immune response. * **Option B:** NK cells are the first line of defense against viral infections. They provide immediate non-specific cytotoxicity before the adaptive immune system (CD8+ T-cells) can mount a specific response. * **Option C:** NK cells play a crucial role in "immunosurveillance." They identify and destroy malignant cells that display stress-induced ligands or abnormal surface proteins. **High-Yield Clinical Pearls for NEET-PG:** * **Origin:** NK cells are large granular lymphocytes derived from the lymphoid progenitor but do not express T-cell receptors (TCR) or CD3. * **Markers:** The definitive surface markers for NK cells are **CD56** and **CD16** (FcγRIII). CD16 allows them to perform **Antibody-Dependent Cellular Cytotoxicity (ADCC)**. * **Mechanism of Killing:** They induce apoptosis in target cells using **perforins** (to create pores) and **granzymes** (to activate caspases). * **Receptors:** They possess Killer Immunoglobulin-like Receptors (KIRs). Inhibitory KIRs bind to MHC I on healthy cells to prevent "self" destruction.

Question 277: What is true about the WIC (Western Immunogenetics Conference) complex?

A. It is present on chromosome 4.
B. Class II comprises A, B, and C loci.
C. Class III has complement components. (Correct Answer)
D. Class I is involved in the mixed leukocyte reaction.

Explanation: The question refers to the **Major Histocompatibility Complex (MHC)**, often discussed in the context of immunogenetics. The MHC is a cluster of genes essential for the immune system to recognize foreign molecules. ### **Explanation of Options** * **Correct Answer: C. Class III has complement components.** The MHC locus is divided into three classes. **Class III genes** are located between Class I and Class II regions. Unlike Class I and II, they do not encode antigen-presenting molecules. Instead, they encode various secreted proteins involved in the immune response, most notably **complement components (C2, C4A, C4B)** and **Factor B**, as well as cytokines like TNF-α and TNF-β. * **A is incorrect:** The MHC (Human Leukocyte Antigen or HLA system in humans) is located on the short arm of **Chromosome 6**, not chromosome 4. * **B is incorrect:** **Class I** comprises the A, B, and C loci. **Class II** comprises the **DP, DQ, and DR** loci. * **D is incorrect:** The **Mixed Leukocyte Reaction (MLR)** is primarily a measure of **Class II MHC** incompatibility (specifically HLA-DR). Class I molecules are involved in presenting endogenous antigens to CD8+ T-cells. ### **High-Yield Clinical Pearls for NEET-PG** * **MHC Restriction:** CD8+ T-cells are MHC Class I restricted, while CD4+ T-cells are MHC Class II restricted (Rule of 8: 8×1=8; 4×2=8). * **Structure:** Class I has one heavy chain and a **β2-microglobulin** (encoded on Chromosome 15). Class II consists of two polypeptide chains (α and β), both encoded within the MHC locus. * **Ankylosing Spondylitis:** Strongly associated with **HLA-B27** (Class I). * **Narcolepsy:** Strongly associated with **HLA-DR2/DQB1** (Class II).

Question 278: Nude mice are able to accept xenografts because they lack:

A. T cells (Correct Answer)
B. B-cells
C. NK cells
D. LAK cells

Explanation: **Explanation:** The **Nude Mouse** is a laboratory strain characterized by a genetic mutation in the *FOXN1* gene. This mutation results in two primary phenotypic features: the absence of hair (hence "nude") and a **congenital absence of the thymus (thymic aplasia)**. 1. **Why T cells is correct:** The thymus is the primary site for T-cell maturation. Without a functional thymus, nude mice cannot produce mature, functional T lymphocytes. Since **T cells (specifically Cytotoxic T cells)** are the primary mediators of Cell-Mediated Immunity (CMI) and are responsible for the rejection of foreign tissue (allografts and xenografts), their absence allows these mice to accept grafts even from different species (xenografts). 2. **Why other options are incorrect:** * **B-cells:** Nude mice possess a normal population of B-cells. While their antibody response to T-dependent antigens is impaired, the B-cells themselves are present. * **NK cells:** Natural Killer (NK) cell levels are actually **normal or even elevated** in nude mice. They provide a baseline level of innate immunity against tumors and infections. * **LAK cells:** Lymphokine-activated killer cells are derived primarily from NK cells in the presence of IL-2. Since NK cells are present, the potential for LAK cell activity remains. **High-Yield Clinical Pearls for NEET-PG:** * **DiGeorge Syndrome:** The human clinical counterpart to the nude mouse, characterized by thymic hypoplasia and T-cell deficiency. * **Xenograft:** A transplant between different species (e.g., pig to human). * **Role in Research:** Nude mice are extensively used in oncology to grow human tumor explants (xenografts) because their lack of T-cell-mediated rejection prevents them from destroying the human cancer cells.

Question 279: Which of the following are examples of live vaccines?

A. Rubella and Yellow fever (Correct Answer)
B. Polio and TAB
C. Diphtheria and Tetanus
D. Hepatitis A and Rabies

Explanation: **Explanation:** Live attenuated vaccines contain pathogens that have been weakened (attenuated) in a laboratory so they can replicate and induce a robust immune response without causing the actual disease in healthy individuals. **1. Why Option A is Correct:** Both **Rubella** and **Yellow fever** are classic examples of live attenuated viral vaccines. Rubella uses the **RA 27/3 strain**, while Yellow fever uses the **17D strain**. These vaccines provide long-lasting immunity, often with a single dose, by mimicking a natural infection. **2. Why Other Options are Incorrect:** * **Option B:** While Oral Polio Vaccine (Sabin) is live, **TAB** (Typhoid, Paratyphoid A and B) is a killed whole-cell vaccine (now largely replaced by newer typhoid vaccines). Note that Injectable Polio Vaccine (Salk) is also killed. * **Option C:** **Diphtheria and Tetanus** are **Toxoid** vaccines. They are prepared by detoxifying bacterial exotoxins using formaldehyde, inducing immunity against the toxin rather than the bacteria itself. * **Option D:** **Hepatitis A** and **Rabies** are **Killed (Inactivated)** vaccines. They contain pathogens that have been destroyed by heat or chemicals and cannot replicate. **Clinical Pearls for NEET-PG:** * **Mnemonic for Live Vaccines:** "**BOY** **R**omeo **I**s **V**ery **M**uch **L**oving **T**yping **I**n **S**mart **P**hone" (**B**CG, **O**PV, **Y**ellow Fever, **R**otavirus, **I**nfluenza (Intranasal), **V**aricella, **M**easles, **M**umps, **L**ive Typhoid (Ty21a), **S**mallpox). * **Contraindication:** Live vaccines are generally contraindicated in **pregnancy** and **immunocompromised** states (e.g., HIV with low CD4 count) due to the risk of uncontrolled viral replication. * **Yellow Fever:** It is a mandatory vaccine for international travel to endemic zones; immunity starts after 10 days and lasts for life.

Question 280: All of the following are disorders of phagocyte function, except?

A. Chronic granulomatous disease
B. X-linked SCID (Correct Answer)
C. Chediak-Higashi syndrome
D. Myeloperoxidase deficiency

Explanation: ### Explanation The correct answer is **B. X-linked SCID**. #### 1. Why X-linked SCID is the correct answer **Severe Combined Immunodeficiency (SCID)** is a disorder of **adaptive immunity**, not phagocyte function. Specifically, X-linked SCID is caused by a mutation in the **IL-2 receptor gamma chain ($\gamma$c)**. This defect leads to a failure in the signaling of multiple interleukins (IL-2, 4, 7, 9, 15, and 21), resulting in a profound deficiency of both **T-cells and NK-cells**, with a secondary dysfunction of B-cells. #### 2. Analysis of Phagocyte Function Disorders (Incorrect Options) The other options represent classic defects in the innate immune system’s phagocytic pathway: * **Chronic Granulomatous Disease (CGD):** A defect in the **NADPH oxidase enzyme** complex. Phagocytes can ingest bacteria but cannot produce the "respiratory burst" (superoxide radicals) needed to kill catalase-positive organisms. * **Chediak-Higashi Syndrome:** A defect in the **LYST gene** (lysosomal trafficking regulator). This leads to a failure in phagosome-lysosome fusion and the presence of giant intracytoplasmic granules in neutrophils. * **Myeloperoxidase (MPO) Deficiency:** The most common primary phagocyte defect. It involves a deficiency in the enzyme that converts hydrogen peroxide to hypochlorous acid (HOCl), impairing intracellular killing. #### 3. NEET-PG High-Yield Pearls * **CGD Diagnosis:** The **Nitroblue Tetrazolium (NBT) dye reduction test** (negative in CGD) or the more modern **Dihydrorhodamine (DHR) flow cytometry** test. * **Chediak-Higashi Triad:** Partial albinism, recurrent pyogenic infections, and peripheral neuropathy. * **SCID Presentation:** "Bubble boy" disease; presents with failure to thrive, chronic diarrhea, and opportunistic infections (e.g., *Pneumocystis jirovecii*) within the first few months of life. * **Leukocyte Adhesion Deficiency (LAD):** Another high-yield phagocyte disorder characterized by **delayed separation of the umbilical cord** and lack of pus formation.

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Cells and Organs of Immune System

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Antigens and Antibodies

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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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Immunology — MCQs

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