Immunology — MCQs

Immunology Indian Medical PG Practice Questions and MCQs

Question 701: What differentiates NK cells from cytotoxic T cells?

A. Interferons reduce NK cell activity.
B. Antibody specificity. (Correct Answer)
C. Receptor for IgG.
D. Presence in the spleen.

Explanation: **Explanation:** The fundamental difference between Natural Killer (NK) cells and Cytotoxic T cells ($CD8^+$) lies in their **recognition mechanism**. 1. **Why "Antibody Specificity" is correct:** Cytotoxic T cells are part of the **adaptive immune system**. They possess T-cell receptors (TCRs) that recognize specific antigens presented by MHC Class I molecules. In contrast, NK cells are part of the **innate immune system**. They do not possess antigen-specific receptors (like TCRs or antibodies) and do not require prior sensitization. Instead, they use a "balance of signals" from **Killer Immunoglobulin-like Receptors (KIRs)** to identify and kill cells that lack MHC Class I (the "missing self" hypothesis). 2. **Analysis of Incorrect Options:** * **A. Interferons:** Incorrect. Interferons (especially IFN-α and IFN-β) actually **enhance** NK cell activity, making them more effective at killing virally infected cells. * **C. Receptor for IgG:** Incorrect. Both cells can interact with antibodies, but NK cells characteristically express **CD16** (FcγRIII), which allows them to perform Antibody-Dependent Cellular Cytotoxicity (ADCC). This is a feature, not a differentiator that favors T-cells. * **D. Presence in the spleen:** Incorrect. Both NK cells and T cells are found in the spleen and other secondary lymphoid organs. **High-Yield Clinical Pearls for NEET-PG:** * **NK Cell Markers:** CD16 (Fc receptor) and CD56 (NCAM) are the definitive markers. * **MHC Restriction:** $CD8^+$ T cells are MHC-restricted (require MHC-I), while NK cells are **MHC-unrestricted** (kill cells that downregulate MHC-I to evade T cells). * **Granules:** Both use Perforins and Granzymes to induce apoptosis in target cells. * **IL-2 and IL-12:** These cytokines are potent activators of NK cells.

Question 702: Springcatarrh is which type of hypersensitivity reaction?

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

Explanation: **Explanation:** **Spring Catarrh**, also known as **Vernal Keratoconjunctivitis (VKC)**, is primarily a **Type I (Immediate) Hypersensitivity reaction**. It is an allergic ocular condition typically seen in young males living in hot, dry climates. The pathophysiology involves the degranulation of mast cells in the conjunctiva upon exposure to environmental allergens (like pollen), leading to the release of histamine and other inflammatory mediators. This results in the classic symptoms of intense itching, photophobia, and ropy discharge. **Analysis of Options:** * **Type I (Correct):** VKC is IgE-mediated. The presence of eosinophils in conjunctival scrapings and elevated serum IgE levels confirms this mechanism. * **Type II (Incorrect):** This involves cytotoxic antibodies (IgG/IgM) against cell surface antigens (e.g., Goodpasture syndrome). VKC does not involve direct cell lysis by antibodies. * **Type III (Incorrect):** This is mediated by immune-complex deposition (e.g., SLE, Arthus reaction). While some chronic components of VKC may involve complex pathways, it is not the primary classification. * **Type IV (Incorrect):** This is a delayed-type hypersensitivity mediated by T-cells (e.g., Contact dermatitis). While some researchers suggest a Th2-driven delayed component in VKC, for examination purposes, it is classified as Type I. **High-Yield Clinical Pearls for NEET-PG:** * **Hallmark Sign:** **Cobblestone papillae** (giant papillae) on the superior palpebral conjunctiva. * **Trantas Dots:** White limbal dots consisting of eosinophils and epithelial debris. * **Shield Ulcer:** A sterile, transverse oval corneal ulcer seen in severe cases. * **Cytology:** Conjunctival scraping shows an abundance of **Eosinophils**.

Question 703: MHC class II molecules are typically expressed on which of the following cell types?

A. Platelets
B. Red blood cells
C. Macrophages (Correct Answer)
D. Endothelial cells

Explanation: **Explanation:** The Major Histocompatibility Complex (MHC) molecules are critical for the immune system to distinguish self from non-self. **MHC Class II** molecules are specifically expressed on **Professional Antigen-Presenting Cells (pAPCs)**. **1. Why Macrophages are correct:** Macrophages, along with Dendritic cells and B-cells, are the primary pAPCs. Their role is to engulf exogenous pathogens, process them into peptides, and present them via MHC Class II molecules to **CD4+ T-helper cells**. This interaction is the "first signal" required to initiate an adaptive immune response. **2. Why the other options are incorrect:** * **Platelets and Red Blood Cells (RBCs):** These are non-nucleated cells. MHC Class I is expressed on almost all nucleated cells; therefore, RBCs lack both MHC I and MHC II. Platelets also do not typically express MHC II. * **Endothelial Cells:** While they are nucleated and express **MHC Class I**, they do not constitutively express MHC Class II. They may express it only under specific inflammatory conditions (induced by Interferon-gamma), but they are not considered "professional" APCs. **High-Yield Clinical Pearls for NEET-PG:** * **MHC Restriction:** MHC I presents to CD8+ T-cells (Rule of 8: 1 × 8 = 8), while MHC II presents to CD4+ T-cells (Rule of 8: 2 × 4 = 8). * **Structure:** MHC II is a heterodimer of **α and β chains**, both encoded by the MHC locus (HLA-DP, DQ, DR). * **Invariant Chain (Ii):** This protein binds to the MHC II groove in the ER to prevent binding of self-peptides before it reaches the endosome. * **Human Analogue:** In humans, MHC is synonymous with **HLA (Human Leukocyte Antigen)**.

Question 704: Which of the following acts as the primary mediator for T-cell independent antigen response?

A. T-cells
B. B-cells (Correct Answer)
C. Macrophages
D. CD8+ T cells

Explanation: ### Explanation **Correct Answer: B. B-cells** Antigens are classified into two types based on their requirement for T-cell assistance to induce an antibody response: **T-dependent (TD)** and **T-independent (TI)**. **T-independent (TI) antigens** (e.g., bacterial polysaccharides, lipopolysaccharides) are typically non-protein molecules with repeating epitopes. These antigens can directly cross-link multiple **B-cell receptors (BCRs)** on the surface of a B-cell. This cross-linking provides a signal strong enough to activate the **B-cell** directly, leading to its proliferation and differentiation into plasma cells without the help of T-helper cells. This is the primary mechanism for the body's rapid response against encapsulated bacteria. **Analysis of Incorrect Options:** * **A. T-cells:** By definition, T-independent responses bypass the need for T-cell involvement. T-cells are essential for T-dependent antigens (mostly proteins) where MHC II presentation is required. * **C. Macrophages:** While macrophages act as Antigen Presenting Cells (APCs) for T-dependent responses, they are not the primary mediators of the antibody-producing response in TI pathways. * **D. CD8+ T cells:** These are cytotoxic T cells involved in cell-mediated immunity (killing virally infected or tumor cells) and do not mediate the humoral (antibody) response to TI antigens. **High-Yield Clinical Pearls for NEET-PG:** * **Antibody Class:** TI antigens primarily produce **IgM** antibodies. They generally do not induce isotype switching (to IgG/IgA) or significant immunological memory. * **Clinical Relevance:** Patients with **splenectomy** or **Wiskott-Aldrich syndrome** have impaired responses to TI-2 antigens (polysaccharides), making them highly susceptible to encapsulated organisms like *S. pneumoniae* and *H. influenzae*. * **Vaccine Design:** To make polysaccharide vaccines (TI) more effective in infants (whose TI response is immature), they are **conjugated** to a protein carrier to convert them into T-dependent antigens.

Question 705: Which of the following does NOT show antitumor activity?

A. Cytotoxic T lymphocytes
B. Natural killer cells
C. Basophils (Correct Answer)
D. Macrophages

Explanation: **Explanation:** The immune system employs a mechanism known as **immunosurveillance** to identify and eliminate nascent tumor cells. This process primarily involves cell-mediated immunity. **Why Basophils are the Correct Answer:** Basophils are granulocytes primarily involved in **Type I hypersensitivity reactions** (allergic responses) and defense against helminthic parasites. They circulate in the blood and release mediators like histamine and heparin upon activation. Unlike other leukocytes, basophils do **not** possess direct tumoricidal activity or the ability to recognize tumor-associated antigens. **Analysis of Incorrect Options:** * **Cytotoxic T Lymphocytes (CTLs/CD8+):** These are the primary effectors of antitumor immunity. They recognize tumor-specific antigens presented on MHC Class I molecules and induce apoptosis via perforins and granzymes. * **Natural Killer (NK) Cells:** These are the "first line of defense" against tumors. They are particularly effective against tumor cells that attempt to evade CTLs by downregulating MHC Class I expression (the "missing self" hypothesis). * **Macrophages:** Activated macrophages (specifically the M1 phenotype) can kill tumor cells by releasing Reactive Oxygen Species (ROS), Nitric Oxide (NO), and Tumor Necrosis Factor (TNF-α). They also act as professional antigen-presenting cells to prime T-cell responses. **NEET-PG High-Yield Pearls:** * **Most potent killer of tumor cells:** Cytotoxic T Lymphocytes (CD8+). * **Cytokine with maximum antitumor activity:** Interferon-gamma (IFN-γ) and TNF-α. * **M1 vs. M2 Macrophages:** M1 macrophages are **antitumor** (pro-inflammatory), while M2 macrophages (Tumor-Associated Macrophages) can actually **promote** tumor growth and angiogenesis. * **LAK Cells:** Lymphokine-activated killer cells (NK cells treated with IL-2) are used in experimental immunotherapy.

Question 706: Which organs are typically affected in Graft versus Host reaction?

A. Skin
B. Gastrointestinal tract
C. Liver
D. All of the above (Correct Answer)

Explanation: **Explanation** **Graft-versus-Host Disease (GVHD)** occurs when immunocompetent T-cells from a donor graft (the "graft") recognize the recipient (the "host") as foreign and initiate an immune attack. This typically occurs in bone marrow or stem cell transplant recipients who are immunocompromised. **Why "All of the above" is correct:** The pathophysiology of GVHD involves the activation of donor T-cells, which migrate to specific target organs. The "classic triad" of organs affected in GVHD includes: 1. **Skin (Option A):** Usually the first organ affected, presenting as a maculopapular rash, often starting on the palms, soles, and neck. 2. **Gastrointestinal Tract (Option B):** Presents with profuse watery diarrhea, abdominal pain, and mucosal ulceration. 3. **Liver (Option C):** Characterized by cholestatic jaundice and elevated alkaline phosphatase due to damage to the bile duct epithelium. **Why other options are incorrect:** Options A, B, and C are individual components of the systemic reaction. Selecting only one would be incomplete, as GVHD is a multisystem disorder where these three systems are the primary targets. **NEET-PG High-Yield Pearls:** * **Prerequisites (Billingham’s Criteria):** 1. The graft must contain immunologically competent cells. 2. The recipient must possess antigens lacking in the donor. 3. The recipient must be incapable of mounting an effective response to destroy the graft. * **Acute vs. Chronic:** Acute GVHD occurs within **100 days** of transplant; Chronic GVHD occurs after 100 days and mimics autoimmune diseases like Scleroderma or Sjögren’s syndrome. * **Prevention:** Depletion of T-cells from the donor graft and use of immunosuppressants like Cyclosporine or Methotrexate.

Question 707: A woman with infertility receives an ovary transplant from her sister who is an identical twin. What type of graft is it?

A. Allograft
B. Autograft
C. Xenograft
D. Isograft (Correct Answer)

Explanation: ### Explanation The correct answer is **Isograft (Option D)**. **1. Why Isograft is correct:** An **Isograft** (also known as a syngeneic graft) is a tissue or organ transplant between two genetically identical individuals of the same species. In humans, this occurs exclusively between **monozygotic (identical) twins**. Because the donor and recipient share identical Major Histocompatibility Complex (MHC/HLA) molecules, the recipient’s immune system does not recognize the graft as foreign. Consequently, there is no immune response, and the graft is accepted without the need for long-term immunosuppression. **2. Why other options are incorrect:** * **Allograft (A):** This is a transplant between genetically different members of the same species (e.g., non-identical siblings, parent-to-child, or unrelated donors). This is the most common type of clinical transplant and requires immunosuppression to prevent rejection. * **Autograft (B):** This involves moving tissue from one site to another on the **same individual** (e.g., a skin graft from the thigh to the arm or a CABG using the saphenous vein). * **Xenograft (C):** This is a transplant between members of **different species** (e.g., a porcine/pig heart valve transplanted into a human). These are subject to rapid hyperacute rejection. **3. NEET-PG High-Yield Pearls:** * **Order of Immunogenicity:** Xenograft > Allograft > Isograft = Autograft. * **MHC/HLA:** The primary barrier to transplantation is the MHC (HLA in humans) located on **Chromosome 6**. * **Identical Twins vs. Fraternal Twins:** A transplant between fraternal (dizygotic) twins is considered an **Allograft**, not an isograft, because they are genetically distinct. * **Clinical Note:** Isografts and Autografts do not require immunosuppressive therapy because they do not trigger T-cell mediated rejection.

Question 708: Which region of the lymph node is T cell dependent?

A. Cortical follicles of lymph node
B. Medullary cords
C. Mantle layer
D. Paracortical area (Correct Answer)

Explanation: **Explanation:** The lymph node is anatomically divided into three main zones: the cortex, the paracortex, and the medulla. Each zone houses specific immune cell populations. **1. Why Paracortical Area is Correct:** The **paracortex** is the **T-cell dependent zone** of the lymph node. It is located between the outer cortex and the inner medulla. This region contains High Endothelial Venules (HEVs), through which T-cells enter from the blood. It also houses Dendritic Cells that present antigens to T-cells to initiate cell-mediated immune responses. In conditions like DiGeorge Syndrome (T-cell deficiency), this specific area is poorly developed. **2. Analysis of Incorrect Options:** * **Cortical follicles (Option A):** These are **B-cell dependent zones**. Primary follicles contain resting B-cells, while secondary follicles contain germinal centers where B-cells proliferate after antigenic stimulation. * **Medullary cords (Option B):** These are located in the innermost part of the node and primarily contain **plasma cells** (secreting antibodies), macrophages, and B-cells. * **Mantle layer (Option C):** This is the outer ring of a secondary follicle consisting of resting, small B-lymphocytes. It is a B-cell predominant region. **High-Yield Clinical Pearls for NEET-PG:** * **B-cell Zone:** Cortex (Follicles/Germinal Centers). * **T-cell Zone:** Paracortex. * **DiGeorge Syndrome:** Paracortex is hypocellular/depleted. * **Agammaglobulinemia (Bruton’s):** Germinal centers and follicles are absent or poorly developed. * **Lymph Node Drainage:** Afferent lymphatics → Subcapsular sinus → Cortical sinus → Medullary sinus → Efferent lymphatics.

Question 709: Prokaryotes have all the following structures except?

A. Rigid cell wall
B. Flagella
C. Ribosomes
D. Well-defined nuclear membrane (Correct Answer)

Explanation: ### Explanation The fundamental distinction between prokaryotes (e.g., bacteria) and eukaryotes (e.g., fungi, protozoa, human cells) lies in their cellular organization. **Why Option D is Correct:** Prokaryotes (from Greek *pro* = before; *karyon* = nucleus) lack a **well-defined nuclear membrane**. Their genetic material (a single, circular double-stranded DNA) is located in an irregular region called the **nucleoid**, which is not separated from the cytoplasm by a membrane. Additionally, prokaryotes lack membrane-bound organelles like mitochondria, Golgi bodies, and lysosomes. **Analysis of Incorrect Options:** * **A. Rigid cell wall:** Most prokaryotes possess a rigid cell wall composed of **peptidoglycan** (murein), which provides structural integrity and protects against osmotic lysis. (Exception: *Mycoplasma* lacks a cell wall). * **B. Flagella:** Many bacteria use flagella for motility. Unlike eukaryotic flagella (9+2 microtubule arrangement), prokaryotic flagella are made of the protein **flagellin** and move in a rotary fashion. * **C. Ribosomes:** Prokaryotes contain **70S ribosomes** (composed of 30S and 50S subunits) for protein synthesis. These are smaller than the 80S ribosomes found in the eukaryotic cytoplasm. **High-Yield Clinical Pearls for NEET-PG:** * **Antibiotic Target:** The difference in ribosome size (70S vs. 80S) is the basis for the selective toxicity of antibiotics like Aminoglycosides and Macrolides. * **Sterols:** Prokaryotic cell membranes lack sterols (except *Mycoplasma*, which incorporates host cholesterol). * **Extrachromosomal DNA:** Prokaryotes often contain **plasmids**, which carry genes for antibiotic resistance (R-factors) and virulence. * **Mesosomes:** These are invaginations of the plasma membrane in bacteria that function similarly to mitochondria (site of respiration).

Question 710: The Prausnitz-Kustner (PK) reaction was used to demonstrate which immunoglobulin?

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

Explanation: ### Explanation **Correct Option: D (IgE)** The **Prausnitz-Küstner (PK) reaction** is a classic immunological test used to demonstrate **Type I Hypersensitivity** and the presence of **reaginic antibodies (IgE)**. In 1921, Küstner (who was allergic to fish) injected his serum into the skin of Prausnitz (who was not). When the fish antigen was later injected into the same site on Prausnitz, a wheal-and-flare reaction occurred. This proved that a specific serum factor—later identified as **IgE**—is responsible for immediate hypersensitivity by binding to mast cells via its Fc portion (homocytotropism). **Why Incorrect Options are Wrong:** * **IgA (Option A):** Primarily involved in mucosal immunity and found in secretions (tears, saliva, colostrum). It does not mediate Type I hypersensitivity. * **IgG (Option B):** The most abundant circulating antibody; it crosses the placenta and mediates Type II and III hypersensitivity, but it is not the primary mediator of the PK reaction. * **IgM (Option C):** The first antibody produced in a primary immune response and a potent activator of the classical complement pathway. It does not bind to mast cells to trigger the PK reaction. **High-Yield Clinical Pearls for NEET-PG:** * **Heat Lability:** IgE is the only immunoglobulin that is **heat-labile** (inactivated at 56°C for 30 minutes). * **Homocytotropism:** IgE is "homocytotropic," meaning it has a high affinity for receptors on mast cells and basophils of the same species. * **Safety Note:** The PK reaction is no longer used clinically due to the risk of transmitting blood-borne pathogens like Hepatitis B and HIV. * **Casoni’s Test:** Another example of an immediate hypersensitivity (Type I) skin test used for Hydatid disease.

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Immunology — MCQs

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