Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 831: A 45-year-old businesswoman arrives with vague abdominal complaints and has noticed melenic stool. A sigmoidoscopy reveals a 4-cm mass in the upper colon. Which tumor marker should be ordered?

A. a-Fetoprotein
B. Anti-tumor antibody
C. Antitumor light chains
D. Carcinoembryonic antigen (CEA) (Correct Answer)

Explanation: **Explanation:** The clinical presentation of a 45-year-old with vague abdominal symptoms, melena (indicating gastrointestinal bleeding), and a 4-cm colonic mass is highly suggestive of **Colorectal Carcinoma (CRC)**. **Why Carcinoembryonic Antigen (CEA) is correct:** CEA is an oncofetal glycoprotein normally produced during fetal development and found in very low levels in adults. It is the most widely used tumor marker for **adenocarcinomas of the colon**. While it lacks the sensitivity and specificity required for primary screening, it is the gold standard for **monitoring treatment response** and detecting **post-operative recurrence** in CRC patients. **Analysis of Incorrect Options:** * **A. $\alpha$-Fetoprotein (AFP):** This is the primary marker for **Hepatocellular Carcinoma (HCC)** and non-seminomatous germ cell tumors (e.g., Yolk sac tumors). * **B. Anti-tumor antibody:** While the immune system may produce antibodies against tumor antigens, these are not standardized or clinically used as diagnostic markers for colon cancer. * **C. Antitumor light chains:** This likely refers to Bence-Jones proteins (monoclonal free light chains), which are diagnostic markers for **Multiple Myeloma**, not solid gastrointestinal tumors. **NEET-PG High-Yield Pearls:** * **CEA** is also elevated in pancreatic, gastric, and breast cancers, as well as in smokers and patients with inflammatory bowel disease (IBD). * **CA 19-9:** Marker for Pancreatic and Cholangiocarcinoma. * **CA-125:** Marker for Ovarian cancer. * **PSA:** Marker for Prostate cancer. * **Rule of Thumb:** Tumor markers are generally used for **prognosis and monitoring**, not for definitive diagnosis (which requires biopsy).

Question 832: What is the function of T-lymphocytes?

A. Production of interferon
B. Lymphokine production
C. Rosette formation
D. All of the above (Correct Answer)

Explanation: **Explanation:** T-lymphocytes (T-cells) are the primary mediators of **Cell-Mediated Immunity (CMI)**. They originate in the bone marrow and mature in the thymus, playing a central role in orchestrating the immune response. 1. **Lymphokine Production:** Upon activation by antigens presented by MHC molecules, T-helper (CD4+) cells secrete cytokines known as **lymphokines** (e.g., IL-2, IL-4, IFN-γ). These chemical messengers coordinate the activities of B-cells, macrophages, and cytotoxic T-cells. 2. **Production of Interferon:** Specifically, Th1 cells produce **Interferon-gamma (IFN-γ)**. This is a potent activator of macrophages and is crucial for controlling intracellular pathogens like *Mycobacterium tuberculosis*. 3. **Rosette Formation:** This is a classic laboratory characteristic. T-cells possess **CD2 receptors** (LFA-2) that bind to LFA-3 on sheep erythrocytes. When mixed, sheep red blood cells surround the T-cell, forming a "flower-like" pattern known as an **E-rosette**. While largely replaced by flow cytometry, it remains a high-yield diagnostic fact for identifying T-cells in exams. Since T-lymphocytes perform all these functions, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **CD3** is the definitive pan-T cell marker. * **MHC Restriction:** CD4+ cells recognize antigens with MHC II; CD8+ cells recognize antigens with MHC I (Rule of 8: 4×2=8 and 8×1=8). * **Th1 vs Th2:** Th1 cells (IFN-γ, IL-2) drive CMI; Th2 cells (IL-4, IL-5, IL-10) drive humoral immunity and allergic responses. * **Delayed-Type Hypersensitivity (Type IV):** This is primarily mediated by T-lymphocytes and macrophages.

Question 833: Which of the following is characteristic of the mucosal immune system?

A. A vigorous response is made to all nonself antigens encountered
B. Chronic inflammation makes an inhospitable environment for microbes
C. IL-2 and IFN-g contribute to a Th-1 like environment
D. Tolerance to foreign antigens is the norm rather than the exception (Correct Answer)

Explanation: ### Explanation **1. Why the Correct Answer is Right:** The mucosal immune system (MALT) is unique because it is constantly exposed to a massive load of foreign proteins, including commensal bacteria and food antigens. To prevent constant, damaging inflammation, the default state of the mucosal immune system is **immunological tolerance** (oral tolerance). This is mediated by specialized cells like **CD103+ dendritic cells** and **Regulatory T cells (Tregs)**, which secrete anti-inflammatory cytokines like **IL-10** and **TGF-β**. This ensures the body does not mount an unnecessary immune response against harmless substances. **2. Analysis of Incorrect Options:** * **Option A:** If the mucosal system made a vigorous response to *all* nonself antigens, we would suffer from constant systemic inflammation and severe food allergies. The system is designed to be "hyporesponsive" to harmless antigens while remaining "vigilant" against pathogens. * **Option B:** Chronic inflammation is actually a **pathological state** (e.g., Inflammatory Bowel Disease) rather than a characteristic feature. The healthy mucosal surface is characterized by "physiological inflammation"—a controlled, non-destructive presence of immune cells. * **Option C:** The mucosal environment is typically **Th-2 and Th-3/Treg** biased. IL-2 and IFN-γ are pro-inflammatory Th-1 cytokines. In the gut, TGF-β is the dominant cytokine, as it promotes the class switching of B cells to produce **Secretory IgA (sIgA)**. **3. Clinical Pearls for NEET-PG:** * **Secretory IgA (sIgA):** The most abundant immunoglobulin in the mucosal system; it acts via "immune exclusion" (prevents attachment) without triggering the complement cascade. * **M Cells:** Specialized epithelial cells in Peyer’s patches that sample antigens from the lumen via transcytosis. * **Homing:** Mucosal lymphocytes express **α4β7 integrin**, which binds to **MAdCAM-1** on mucosal endothelial cells, ensuring they return to mucosal sites.

Question 834: Which immunoglobulin has a pentameric structure?

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

Explanation: **Explanation:** **IgM** is the correct answer because it is the only immunoglobulin that primarily exists as a **pentamer** in its secreted form. It consists of five basic H2L2 units held together by disulfide bonds and a specialized polypeptide called the **J-chain** (Joining chain). Due to its pentameric structure, it has 10 antigen-binding sites, giving it the highest **valency** and making it highly efficient at agglutination and complement activation via the classical pathway. **Analysis of Incorrect Options:** * **IgG:** The most abundant antibody in serum, it exists strictly as a **monomer**. It is the only antibody that crosses the placenta. * **IgA:** Primarily found in secretions (tears, saliva, colostrum). While it is a monomer in serum, **secretory IgA** exists as a **dimer** (connected by a J-chain and a secretory component). * **IgD:** Found on the surface of B-cells, it exists only as a **monomer** and acts as an antigen receptor. **High-Yield Clinical Pearls for NEET-PG:** * **First Responder:** IgM is the first antibody to appear in response to an initial exposure to an antigen (Primary Immune Response). * **Infection Marker:** Presence of specific IgM indicates a **recent/acute infection**, whereas IgG indicates past infection or chronic state. * **Molecular Weight:** IgM is the largest immunoglobulin ("Millionaire Molecule"), preventing it from crossing the placental barrier. * **Isohemagglutinins:** Naturally occurring anti-A and anti-B antibodies in the ABO blood group system are of the IgM class.

Question 835: Which of the following is NOT a test for B cell function?

A. CH50 count (Correct Answer)
B. Amount of IgG
C. PPD testing
D. Cytokine levels

Explanation: To answer this question, we must distinguish between the components of the immune system: **B cells (Humoral immunity)**, **T cells (Cell-mediated immunity)**, and the **Complement system**. ### **Why CH50 count is the correct answer** The **CH50 (Total Hemolytic Complement) assay** is a screening test used to evaluate the functional integrity of the **Classical Complement Pathway** (C1 through C9). It measures the ability of a patient's serum to lyse 50% of antibody-sensitized sheep erythrocytes. While B cells produce the antibodies that trigger this pathway, the CH50 test specifically assesses complement protein activity, not B cell function itself. ### **Analysis of other options** * **Amount of IgG (Option B):** B cells differentiate into plasma cells which secrete immunoglobulins (IgG, IgA, IgM, etc.). Measuring serum immunoglobulin levels is a direct quantitative assessment of B cell function. * **PPD testing (Option C):** This is a classic example of a **Type IV Hypersensitivity reaction**, which is mediated by **T cells**. *Note: In many standardized formats, PPD is used to test T cell function. If the question asks for "NOT a B cell test," both A and C are technically non-B cell tests; however, CH50 is the most distinct as it tests the innate/complement system.* * **Cytokine levels (Option D):** B cells function as Antigen Presenting Cells (APCs) and produce various cytokines (e.g., IL-6, TNF-alpha) to regulate immune responses. Measuring these can reflect B cell activity. ### **NEET-PG High-Yield Pearls** * **B Cell Function Tests:** Serum electrophoresis, Schick test (evaluates IgG response to diphtheria toxin), and flow cytometry for CD19/CD20 markers. * **T Cell Function Tests:** Skin tests (PPD, Candida), CD3/CD4/CD8 counts, and the Lymphocyte Transformation Test. * **Complement Deficiency:** A low CH50 usually indicates a deficiency in one or more complement components (C1-C9) or consumption due to immune complex diseases like SLE.

Question 836: Which of the following is FALSE about the primary immune response?

A. It is the immune response against subsequent antigenic challenge. (Correct Answer)
B. The lag period is longer.
C. It is slow, sluggish, and short-lived.
D. All of the above.

Explanation: The primary immune response occurs when the body encounters an antigen for the **first time**. Understanding the differences between primary and secondary responses is a high-yield topic for NEET-PG. ### **Explanation of Options** * **Option A (Correct):** This statement is **FALSE**. The immune response against a *subsequent* (second or later) challenge is called the **Secondary Immune Response** (or Anamnestic Response). The primary response is the initial reaction to the first exposure. * **Option B (Incorrect):** This statement is **TRUE**. In a primary response, the lag period (the time before antibodies appear in the blood) is longer, typically lasting **5–10 days**, as it takes time for naive B-cells to undergo clonal expansion and differentiation. * **Option C (Incorrect):** This statement is **TRUE**. Because the body has no "memory" of the pathogen, the response is slow to peak, sluggish in intensity, and the antibody titers decline rapidly (short-lived). ### **High-Yield Clinical Pearls for NEET-PG** | Feature | Primary Response | Secondary Response | | :--- | :--- | :--- | | **Antigen Exposure** | First time | Subsequent (Repeat) | | **Lag Period** | Long (5–10 days) | Short (1–3 days) | | **Predominant Antibody** | **IgM** | **IgG** (predominant), IgA, or IgE | | **Antibody Titer** | Low | Very High | | **Affinity Maturation** | Low affinity | **High affinity** (Affinity Maturation) | | **Memory Cells** | Produced at the end | Already present; rapidly activated | **Key Concept:** The secondary response is faster and more potent due to the presence of **Memory B and T cells** and the process of **Somatic Hypermutation**, which ensures higher antibody affinity.

Question 837: Which cells are primarily involved in humoral immunity?

A. B-cells (Correct Answer)
B. T-cells
C. Helper cells
D. Dendritic cells

Explanation: **Explanation:** **Humoral immunity** is the aspect of the adaptive immune response mediated by macromolecules found in extracellular fluids (humors), primarily **antibodies**. **Why B-cells are correct:** B-cells are the primary mediators of humoral immunity. Upon encountering a specific antigen and receiving signals from T-helper cells, B-cells differentiate into **plasma cells**. These plasma cells act as "antibody factories," secreting immunoglobulins (IgG, IgM, IgA, IgE, IgD) that neutralize toxins, opsonize pathogens, and activate the complement system. **Why the other options are incorrect:** * **T-cells:** These are the primary mediators of **Cell-Mediated Immunity (CMI)**. They do not produce antibodies but instead destroy infected cells directly or coordinate the immune response. * **Helper cells (CD4+ T-cells):** While they are essential for activating B-cells (T-dependent response), they belong to the cell-mediated arm. They function by secreting cytokines rather than producing antibodies. * **Dendritic cells:** These are professional **Antigen-Presenting Cells (APCs)**. Their primary role is to bridge innate and adaptive immunity by capturing antigens and presenting them to T-cells in the lymph nodes. **NEET-PG High-Yield Pearls:** * **Origin:** Both B and T cells originate in the bone marrow, but B-cells mature in the **bone marrow**, while T-cells mature in the **thymus**. * **Markers:** CD19, CD20, and CD21 are characteristic surface markers for B-cells. * **Memory:** Humoral immunity provides long-term protection through **Memory B-cells**, which is the principle behind most vaccinations. * **Bursa of Fabricius:** In birds, B-cells mature here (hence the name "B" cell); in humans, the functional equivalent is the bone marrow.

Question 838: Which of the following is a chemoattractant?

A. C5a (Correct Answer)
B. Cl
C. C3
D. C2

Explanation: **Explanation:** The complement system is a crucial component of innate immunity, consisting of a cascade of proteins that enhance the ability of antibodies and phagocytic cells to clear pathogens. **Why C5a is Correct:** C5a is a potent **anaphylatoxin** and the most powerful **chemoattractant** of the complement system. During the activation of the complement cascade, C5 is cleaved into C5a and C5b. C5a acts as a chemical signal that recruits neutrophils, monocytes, and macrophages to the site of inflammation (chemotaxis). It also triggers mast cell degranulation, leading to increased vascular permeability. **Why the other options are incorrect:** * **C1:** This is the starting component of the Classical Pathway. It recognizes antigen-antibody complexes but does not possess chemotactic activity. * **C3:** This is the most abundant complement protein. While its cleavage product **C3a** is an anaphylatoxin, C3 itself is an inactive precursor. Its other product, **C3b**, is primarily involved in **opsonization** (tagging pathogens for phagocytosis). * **C2:** This is a component of the Classical and Lectin pathways. Its cleavage products (C2a/C2b) are involved in forming the C3 convertase but do not act as chemoattractants. **High-Yield NEET-PG Pearls:** * **Potency of Chemoattractants:** C5a > LTB4 (Leukotriene B4) > IL-8 > Bacterial products (N-formyl peptides). * **Opsonization:** C3b is the major opsonin. * **Membrane Attack Complex (MAC):** Formed by C5b-C9; responsible for osmotic lysis of gram-negative bacteria. * **Anaphylatoxins:** C5a > C3a > C4a (in order of potency). * **Deficiency:** C5-C9 deficiency increases susceptibility to *Neisseria* infections.

Question 839: EAC rosette formation is a property of which of the following types of immune cells?

A. T-cells (Correct Answer)
B. B-cells
C. Macrophages
D. All of the above

Explanation: **Explanation:** The formation of **rosettes** is a classic laboratory technique used to identify and enumerate specific lymphocyte populations based on their surface receptors. **1. Why T-cells are the correct answer:** T-cells possess a specific surface receptor known as **CD2**, which has a high affinity for **LFA-3** (CD58) found on the surface of sheep erythrocytes (SRBCs). When T-cells are incubated with SRBCs, the red cells adhere to the T-cell surface, forming a cluster that resembles a rose—hence the term **"E-rosette"** (Erythrocyte rosette). This is a gold-standard historical marker for identifying T-lymphocytes. **2. Analysis of Incorrect Options:** * **B-cells:** While B-cells do not form E-rosettes, they can form **EAC rosettes** (Erythrocyte-Antibody-Complement). This occurs because B-cells express receptors for the Fc portion of IgG and the C3b component of complement. However, the standard "rosette formation" mentioned in classical immunology questions specifically refers to the CD2-mediated E-rosette of T-cells. * **Macrophages:** Although macrophages have receptors for complement and Fc fragments, they are primarily identified by phagocytic activity and specific markers like CD14/CD68, not by spontaneous rosette formation with SRBCs. **3. High-Yield Clinical Pearls for NEET-PG:** * **E-Rosette:** Marker for **T-cells** (CD2 receptor). * **EAC-Rosette:** Marker for **B-cells** (CR1/C3b receptor). * **ANAE (Alpha-Naphthyl Acetate Esterase):** A histochemical stain used to differentiate T-cells (focal staining) from B-cells (negative). * **CD3:** The most specific pan-T-cell marker used in modern flow cytometry. * **Surface Immunoglobulin (sIg):** The most definitive marker for B-cells.

Question 840: Omenn syndrome represents which of the following immunological conditions?

A. Human Leukocyte antigen (HLA) Class II Deficiency
B. HLA Class I Deficiency
C. Purine Metabolism Deficiency
D. Subset of T cell deficiency (Correct Answer)

Explanation: **Explanation:** **Omenn Syndrome** is a rare, autosomal recessive form of **Severe Combined Immunodeficiency (SCID)**. It is primarily caused by hypomorphic (partial) mutations in the **RAG1 or RAG2 genes**. 1. **Why the correct answer is right:** In Omenn Syndrome, the RAG gene mutations lead to limited V(D)J recombination. This results in a profound deficiency of B cells, but a **paradoxical expansion of a few restricted, oligoclonal T-cell subsets**. These T cells are poorly regulated, autoreactive, and infiltrate peripheral tissues (skin, gut), leading to the characteristic clinical presentation of erythroderma and hepatosplenomegaly. Thus, it is classified as a specific subset of T-cell deficiency/dysfunction. 2. **Why the incorrect options are wrong:** * **HLA Class II Deficiency (Bare Lymphocyte Syndrome Type II):** Caused by mutations in transcription factors (like CIITA) required for HLA Class II expression. It presents with a lack of CD4+ T cells, not the oligoclonal T-cell expansion seen in Omenn. * **HLA Class I Deficiency (Bare Lymphocyte Syndrome Type I):** Usually due to TAP1/TAP2 mutations; it presents with chronic respiratory infections and skin ulcers, not SCID-like features. * **Purine Metabolism Deficiency:** Refers to **Adenosine Deaminase (ADA) deficiency**, which causes a complete absence of T, B, and NK cells (T-B-NK- SCID) due to toxic metabolite accumulation. **High-Yield Clinical Pearls for NEET-PG:** * **Classic Triad:** Erythroderma (peeling skin), Hepatosplenomegaly, and Lymphadenopathy. * **Laboratory Hallmarks:** Elevated **IgE** levels, **Eosinophilia**, and absent B cells. * **Genetics:** Most commonly **RAG1/RAG2** mutations (hypomorphic). * **Treatment:** Hematopoietic stem cell transplant (HSCT) is the definitive cure.

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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

Immunology — MCQs

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