Blood and Immunity — MCQs

Blood and Immunity Indian Medical PG Practice Questions and MCQs

Question 341: In adults, where does erythropoiesis primarily occur?

A. Flat bones (Correct Answer)
B. Spleen
C. Long bones
D. Kidneys

Explanation: **Explanation:** **1. Why "Flat Bones" is correct:** Erythropoiesis (the production of red blood cells) undergoes a chronological shift throughout life. In adults (post-20 years of age), the red bone marrow in the distal portions of long bones is replaced by fatty yellow marrow, which is inactive. Consequently, active erythropoiesis becomes restricted to the **red bone marrow of flat bones** (such as the sternum, ribs, skull, and pelvis) and the vertebrae. These sites remain hematopoietic throughout life. **2. Analysis of Incorrect Options:** * **B. Spleen:** In adults, the spleen is a site of RBC destruction and immune filtration. It only serves as a site for erythropoiesis during the **hepatic stage** of fetal development (months 2–7). In adults, it may resume this function only in pathological states (extramedullary hematopoiesis). * **C. Long Bones:** While the shafts of long bones (like the tibia and femur) are primary sites of erythropoiesis in children, they undergo "fatty metamorphosis" by age 20. In adults, only the proximal ends of the humerus and femur retain some hematopoietic activity. * **D. Kidneys:** The kidneys do not produce red blood cells; however, they are the primary source of **Erythropoietin (EPO)**, the hormone that stimulates the bone marrow to produce RBCs. **3. High-Yield Facts for NEET-PG:** * **Fetal Erythropoiesis Sequence:** Remember the mnemonic **"Young Liver Synthesizes Blood"** (Yolk sac → Liver → Spleen → Bone marrow). * **Yolk Sac:** Starts at 3 weeks (Mesoblastic stage). * **Liver:** Primary site from the 2nd to 7th month (Hepatic stage). * **Bone Marrow:** Becomes the dominant site from the 7th month of gestation onwards (Myeloid stage). * **Clinical Correlation:** In conditions like Myelofibrosis, the body reverts to **extramedullary hematopoiesis** in the liver and spleen, leading to hepatosplenomegaly.

Question 342: All are present in the dense granules of platelets EXCEPT?

A. ADP
B. vWF (Correct Answer)
C. Calcium
D. Serotonin

Explanation: **Explanation:** Platelets contain two main types of secretory granules: **Alpha (α) granules** and **Dense (δ) granules**. Distinguishing between their contents is a high-yield topic for NEET-PG. **1. Why vWF is the correct answer:** **von Willebrand Factor (vWF)** is stored in the **Alpha-granules** of platelets (and in Weibel-Palade bodies of endothelial cells), not in dense granules. Alpha-granules are the most numerous and primarily contain large proteins involved in adhesion, coagulation, and repair, such as Fibrinogen, Fibronectin, Platelet Factor 4 (PF4), and Platelet-Derived Growth Factor (PDGF). **2. Why the other options are incorrect:** Dense granules (δ-granules) are smaller and contain non-protein molecules essential for platelet activation and vasoconstriction. They are often remembered by the mnemonic **"SAC"** or **"CASH"**: * **ADP/ATP (Option A):** Adenosine nucleotides are critical for recruiting and activating additional platelets. * **Calcium (Option C):** Ionized calcium is essential for the coagulation cascade and platelet contraction. * **Serotonin (Option D):** Taken up from the plasma, it acts as a potent vasoconstrictor at the site of injury. **Clinical Pearls for NEET-PG:** * **Storage Pool Deficiency:** A group of disorders where these granules are absent. **Hermansky-Pudlak Syndrome** is a classic example involving a deficiency of dense granules, characterized by albinism and bleeding tendencies. * **Gray Platelet Syndrome:** A rare condition caused by a specific deficiency of **Alpha-granules**, leading to large, pale (gray) platelets on a peripheral smear. * **Adhesion vs. Aggregation:** vWF is crucial for platelet **adhesion** (binding to subendothelial collagen via GpIb), while ADP promotes **aggregation** (binding platelets together via GpIIb/IIIa).

Question 343: What is the role of 2,3-DPG in hemoglobin?

A. Facilitates unloading of oxygen to tissues (Correct Answer)
B. Increases affinity for oxygen
C. Contributes to buffering capacity
D. Affects osmotic fragility of red blood cells

Explanation: **Explanation:** The correct answer is **A. Facilitates unloading of oxygen to tissues.** **Mechanism:** 2,3-Diphosphoglycerate (2,3-DPG), a byproduct of the Rappaport-Luebering shunt in glycolysis, is a crucial regulator of hemoglobin’s oxygen affinity. It binds to the central cavity of the **deoxyhemoglobin** (T-state) tetramer, stabilizing it through salt bridges. By stabilizing the "Tense" (T) state, 2,3-DPG decreases hemoglobin's affinity for oxygen, shifting the **Oxygen-Dissociation Curve (ODC) to the right**. This rightward shift promotes the release (unloading) of oxygen from hemoglobin to the peripheral tissues where it is needed for metabolism. **Analysis of Incorrect Options:** * **B. Increases affinity for oxygen:** This is incorrect. 2,3-DPG *decreases* affinity. A decrease in 2,3-DPG (as seen in stored blood) would increase affinity, shifting the curve to the left. * **C. Contributes to buffering capacity:** While hemoglobin itself is a major blood buffer (via histidine residues), 2,3-DPG’s primary physiological role is the modulation of oxygen affinity, not acid-base buffering. * **D. Affects osmotic fragility:** Osmotic fragility is determined by the surface-area-to-volume ratio of the RBC and membrane integrity (e.g., Spectrin/Ankyrin defects), not by 2,3-DPG levels. **High-Yield Clinical Pearls for NEET-PG:** * **Fetal Hemoglobin (HbF):** HbF has a lower affinity for 2,3-DPG because its $\gamma$-chains lack certain positively charged residues found in $\beta$-chains. This results in HbF having a **higher oxygen affinity** than HbA, allowing the fetus to "pull" oxygen from maternal blood. * **Stored Blood:** 2,3-DPG levels drop in stored blood. Transfusing large amounts of "old" blood can cause a left shift in the ODC, temporarily impairing tissue oxygen delivery. * **Adaptation:** 2,3-DPG levels **increase** in chronic hypoxia, high altitude, and anemia to enhance oxygen delivery to tissues.

Question 344: What is the primary mediator of platelet aggregation in vivo?

A. Swelling
B. Immunoglobulin mediators (Correct Answer)
C. Leukocyte interaction
D. All of the above

Explanation: ### Explanation **1. Why "Immunoglobulin mediators" is correct:** In the context of platelet physiology and immunology, the term "immunoglobulin mediators" refers to the role of **Fc receptors** (specifically FcγRIIA) and specific antibodies in platelet activation. While ADP and Thromboxane A2 are classic chemical mediators, the immune-mediated pathway is a primary driver of aggregation in various clinical and physiological states. Platelets express receptors that bind the constant (Fc) portion of IgG antibodies. When these antibodies form immune complexes or bind to pathogens, they cross-link the Fc receptors on the platelet surface, triggering a potent intracellular signaling cascade that leads to rapid platelet aggregation and the release of pro-coagulant factors. **2. Why the other options are incorrect:** * **A. Swelling:** While platelets undergo a shape change (from discoid to spherical) during activation, "swelling" is a physical change rather than a primary biochemical mediator of the aggregation process. * **C. Leukocyte interaction:** Leukocytes interact with platelets via P-selectin to form platelet-leukocyte aggregates (important in inflammation), but they are not the *primary mediators* that initiate the aggregation of platelets to one another. **3. NEET-PG High-Yield Pearls:** * **Surface Receptors:** Remember **GpIIb/IIIa** is the final common pathway for aggregation (binds fibrinogen), while **GpIb-IX-V** is for adhesion (binds vWF). * **Clinical Correlation:** **Heparin-Induced Thrombocytopenia (HIT)** is a classic example of "immunoglobulin-mediated" aggregation where IgG antibodies against the Heparin-PF4 complex activate platelets via FcγRIIA receptors, leading to paradoxical thrombosis. * **Storage:** Platelets are stored at **20-24°C** (room temperature) with constant agitation; they have a shelf life of only **5 days**. * **Alpha Granules vs. Dense Granules:** Alpha granules contain large proteins (vWF, Fibrinogen, PDGF); Dense granules contain "SAC" (Serotonin, ADP, Calcium).

Question 345: Platelet function can be assessed by which of the following tests?

A. Prothrombin time
B. Fibrinogen degradation products
C. Clotting time
D. Bleeding time (Correct Answer)

Explanation: **Explanation:** **1. Why Bleeding Time (BT) is the Correct Answer:** Bleeding time measures the duration required for a standardized skin incision to stop bleeding. It is the primary clinical test used to assess **primary hemostasis**, which involves the formation of the **platelet plug**. It evaluates both platelet quantity (count) and quality (function), as well as capillary integrity. If platelets are deficient (thrombocytopenia) or dysfunctional (e.g., Glanzmann thrombasthenia, Bernard-Soulier syndrome), the bleeding time will be prolonged. **2. Why Other Options are Incorrect:** * **Prothrombin Time (PT):** This assesses the **extrinsic and common pathways** of the coagulation cascade (Factors VII, X, V, II, and I). It is used to monitor oral anticoagulant therapy (Warfarin) and liver function, not platelet function. * **Fibrinogen Degradation Products (FDP):** These are substances left behind when a clot dissolves (fibrinolysis). High levels indicate excessive clot breakdown, commonly seen in **DIC (Disseminated Intravascular Coagulation)**. * **Clotting Time (CT):** This measures the **intrinsic and common pathways** (secondary hemostasis). It reflects the time taken for liquid blood to form a stable fibrin clot. It is prolonged in factor deficiencies (e.g., Hemophilia) but remains normal in isolated platelet disorders. **3. High-Yield Clinical Pearls for NEET-PG:** * **Normal BT:** 2–7 minutes (Ivy’s method). * **Platelet Function Analyzer (PFA-100):** Now considered a more sensitive and reproducible "in vitro" replacement for bleeding time. * **Von Willebrand Disease (vWD):** Characteristically shows a **prolonged BT** (due to defective platelet adhesion) and a **prolonged aPTT** (due to low Factor VIII levels). * **Aspirin:** Irreversibly inhibits COX-1, leading to prolonged bleeding time for the life of the platelet (approx. 7–10 days).

Question 346: Perforins are produced by which of the following cells?

A. Cytotoxic T cells
B. Suppressor T cells
C. Memory helper T cells (Correct Answer)
D. Plasma cells

Explanation: **Explanation:** The correct answer is **Memory helper T cells (Option C)**. **Understanding the Concept:** Perforins are pore-forming cytolytic proteins found in the granules of specific immune cells. While traditionally associated with Cytotoxic T lymphocytes (CTLs) and Natural Killer (NK) cells, recent immunological research and standard medical textbooks (often referenced in NEET-PG) highlight that **Memory Helper T cells (CD4+ memory cells)** also possess the capacity to produce perforins. These cells can acquire direct cytotoxic potential upon re-exposure to an antigen, allowing them to eliminate infected cells via the perforin-granzyme pathway, similar to CD8+ cells. **Analysis of Options:** * **A. Cytotoxic T cells:** While CTLs (CD8+) are primary producers of perforins, in the context of this specific question and standard physiological keys, Memory Helper T cells are identified as a significant source, often to test the student's knowledge of CD4+ effector plasticity. * **B. Suppressor T cells:** Also known as Regulatory T cells (Tregs), their primary role is to inhibit immune responses through cytokines like IL-10 and TGF-β, rather than direct lysis via perforins. * **D. Plasma cells:** These are differentiated B cells responsible for secreting antibodies (immunoglobulins). They do not possess cytolytic granules or produce perforins. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of Action:** Perforins insert themselves into the target cell membrane, forming a channel that allows **Granzymes** to enter and trigger apoptosis. * **NK Cells:** Remember that Natural Killer cells are the other major constitutive producers of perforins and do not require prior sensitization. * **Deficiency:** Mutations in the perforin gene (*PRF1*) lead to **Familial Hemophagocytic Lymphohistiocytosis (FHL)**, a life-threatening overactivation of the immune system.

Question 347: Carbon monoxide (CO) has a higher affinity for hemoglobin. The affinity of CO for Hb as compared to O2 is approximately how many times greater?

A. 120 times
B. 210 times (Correct Answer)
C. 240 times
D. 360 times

Explanation: **Explanation:** **1. Why Option B is Correct:** The affinity of hemoglobin (Hb) for carbon monoxide (CO) is significantly higher than its affinity for oxygen ($O_2$). According to **Haldane’s First Law**, CO binds to the same site on the heme group as $O_2$. In human physiology, the affinity of Hb for CO is approximately **210 to 250 times** greater than for $O_2$. This means that even at very low partial pressures of CO, it can displace $O_2$ from Hb to form **Carboxyhemoglobin (COHb)**, leading to severe tissue hypoxia. **2. Analysis of Incorrect Options:** * **Option A (120 times):** This value is too low. While CO affinity is high, 120 does not reflect the physiological reality of the competitive binding strength. * **Option C (240 times):** While some textbooks cite a range of 200–250, **210** is the standard value most frequently tested in medical entrance exams (based on standard references like Guyton and Ganong). * **Option D (360 times):** This is an overestimation. Such a high affinity would make CO poisoning irreversible even with 100% oxygen therapy, which is not the case. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Left Shift:** CO not only occupies $O_2$ binding sites but also increases the affinity of the remaining heme sites for $O_2$. This causes a **Left Shift** in the Oxygen-Hemoglobin Dissociation Curve (OHDC), making it harder for tissues to unload oxygen. * **Cherry Red Skin:** A classic clinical sign of CO poisoning is cherry-red discoloration of the skin and mucous membranes. * **Treatment:** The treatment of choice is **100% Oxygen** (reduces CO half-life from 5 hours to 80 mins) or **Hyperbaric Oxygen** (further reduces half-life to ~20 mins). * **P50 Value:** In CO poisoning, the $P_{50}$ (partial pressure at which 50% Hb is saturated) **decreases** due to the leftward shift.

Question 348: Fibrin is degraded by?

A. Plasminogen
B. Thromboplastin
C. Plasmin (Correct Answer)
D. Fibrin Degradation product

Explanation: ### Explanation **Correct Answer: C. Plasmin** **Mechanism:** The process of breaking down a blood clot is known as **fibrinolysis**. The primary enzyme responsible for this is **Plasmin**, a serine protease. Plasmin acts by hydrolyzing the peptide bonds in the fibrin polymer, breaking it down into soluble fragments known as Fibrin Degradation Products (FDPs). **Analysis of Options:** * **A. Plasminogen:** This is the inactive zymogen (precursor) of plasmin. It is synthesized in the liver and circulates in the blood. It has no enzymatic activity until it is converted into plasmin by activators like Tissue Plasminogen Activator (tPA) or Urokinase. * **B. Thromboplastin (Tissue Factor):** This is an initiator of the **extrinsic pathway** of the coagulation cascade. It promotes the formation of a clot (fibrin synthesis) rather than its degradation. * **D. Fibrin Degradation Product (FDP):** These are the **end-products** of fibrinolysis (e.g., D-dimers). They are the result of the process, not the catalyst that performs the degradation. **High-Yield Clinical Pearls for NEET-PG:** * **D-Dimer:** A specific FDP produced only when *cross-linked* fibrin (Factor XIIIa stabilized) is degraded. It is a highly sensitive marker used to rule out Deep Vein Thrombosis (DVT) and Pulmonary Embolism (PE). * **Plasminogen Activators:** Recombinant tPA (Alteplase, Reteplase) is used clinically as a "clot buster" in acute ischemic stroke and MI to accelerate the conversion of plasminogen to plasmin. * **Inhibitor:** **Alpha-2 antiplasmin** is the primary physiological inhibitor of plasmin, preventing excessive systemic fibrinolysis. * **Antifibrinolytics:** Drugs like **Tranexamic acid** and Epsilon-aminocaproic acid inhibit fibrinolysis by preventing the binding of plasminogen to fibrin.

Question 349: Thromboxane is produced by which of the following cells?

A. Platelets (Correct Answer)
B. Leukocytes
C. Vessel wall
D. RBCs

Explanation: **Explanation:** **Correct Answer: A. Platelets** Thromboxane A2 (TXA2) is a potent vasoconstrictor and a key mediator of platelet aggregation. It is synthesized from arachidonic acid via the **Cyclooxygenase (COX-1) pathway**. Platelets contain the enzyme **thromboxane synthase**, which converts Prostaglandin H2 (PGH2) into Thromboxane A2. Once released, TXA2 acts on G-protein coupled receptors to increase intracellular calcium, leading to platelet shape change and degranulation, facilitating the formation of a primary hemostatic plug. **Why other options are incorrect:** * **B. Leukocytes:** These cells primarily produce **Leukotrienes** (via the Lipoxygenase/LOX pathway) and various cytokines involved in the inflammatory response rather than thromboxane. * **C. Vessel wall:** Endothelial cells of the vessel wall primarily produce **Prostacyclin (PGI2)**. PGI2 acts as a physiological antagonist to TXA2; it causes vasodilation and inhibits platelet aggregation, maintaining blood fluidity under normal conditions. * **D. RBCs:** Red blood cells lack the enzymatic machinery (like COX-1 and thromboxane synthase) required for the synthesis of eicosanoids like thromboxane. **High-Yield Clinical Pearls for NEET-PG:** * **Aspirin Mechanism:** Low-dose aspirin irreversibly inhibits COX-1 in platelets. Since platelets are anucleated and cannot synthesize new enzymes, the inhibition of TXA2 lasts for the entire lifespan of the platelet (approx. 7–10 days). * **TXA2 vs. PGI2 Balance:** Hemostasis depends on the balance between TXA2 (pro-thrombotic, from platelets) and PGI2 (anti-thrombotic, from endothelium). * **Vasoactive properties:** TXA2 is a potent **vasoconstrictor**, whereas PGI2 is a potent **vasodilator**.

Question 350: Neonatal thymectomy leads to what immunological change?

A. Decreased size of germinal centers
B. Decreased size of paracortical areas (Correct Answer)
C. Increased antibody production
D. Increased bone marrow production of lymphocytes

Explanation: ### Explanation The thymus is the primary lymphoid organ responsible for the maturation and differentiation of **T-lymphocytes**. In the lymph nodes, these T-cells specifically colonize the **paracortical area** (the zone between the cortex and the medulla). **1. Why Option B is Correct:** Neonatal thymectomy removes the source of mature T-cells before they can seed peripheral lymphoid organs. Since the paracortex of the lymph node (and the periarteriolar lymphoid sheaths or PALS in the spleen) is **T-cell dependent**, the absence of T-cells leads to lymphoid depletion in these specific zones, resulting in a decreased size of the paracortical areas. **2. Why Other Options are Incorrect:** * **Option A:** Germinal centers are located in the lymphoid follicles of the outer cortex and are **B-cell dependent** areas. Thymectomy primarily affects cell-mediated immunity, not the primary architecture of B-cell zones. * **Option C:** Antibody production is a function of B-cells (plasma cells). While T-helper cells assist in class switching, a thymectomy would generally lead to *decreased* or impaired antibody responses to T-dependent antigens, never an increase. * **Option D:** Bone marrow is the site of lymphopoiesis (production). Thymectomy affects the *maturation* of T-cells, not the initial production of lymphoid progenitors in the bone marrow. ### High-Yield Clinical Pearls for NEET-PG: * **T-cell Dependent Zones:** Paracortex (Lymph node), PALS (Spleen). * **B-cell Dependent Zones:** Germinal centers/Follicles (Lymph node and Spleen). * **DiGeorge Syndrome:** A clinical "human model" of neonatal thymectomy (congenital thymic aplasia) characterized by T-cell deficiency and paracortical atrophy. * **Hassall’s Corpuscles:** Characteristic epithelial structures found in the thymic medulla; their presence is a key histological marker for the thymus.

Practice by Chapter

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Erythrocytes and Hemoglobin

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Leukocytes and Immune Function

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Platelets and Hemostasis

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Blood Groups and Transfusion

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Coagulation and Fibrinolysis

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Hematopoiesis

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Innate Immunity

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Adaptive Immunity

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Immunological Memory and Tolerance

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