Blood and Immunity — MCQs

Blood and Immunity Indian Medical PG Practice Questions and MCQs

Question 141: Blood groups A1 and A2 differ in?

A. Chemical structure of antigen (Correct Answer)
B. Spatial configuration of antigen
C. Molecular groups
D. Ionic charges on surface

Explanation: **Explanation:** The differentiation between blood groups **A1 and A2** is a classic high-yield topic in Immunohematology. While both belong to the ABO system, they differ qualitatively and quantitatively. **1. Why "Chemical structure of antigen" is correct:** The primary difference lies in the **complexity of the carbohydrate chains**. Both A1 and A2 individuals possess the A-transferase enzyme, which adds N-acetylgalactosamine (GalNAc) to the H-substance. However, the **A1 enzyme** is more efficient and can convert both simple and branched precursor chains (Type 1, 2, 3, and 4) into A-antigens. In contrast, the **A2 enzyme** is less efficient and primarily acts on simple, linear chains. This results in a distinct **chemical structure** and a significantly higher density of A-antigen sites on A1 cells (approx. 1 million) compared to A2 cells (approx. 250,000). **2. Why other options are incorrect:** * **Spatial configuration:** This refers to the 3D folding of proteins. Since A-antigens are carbohydrates (glycosphingolipids/glycoproteins), the core difference is the chemical linkage and branching, not just a spatial rearrangement. * **Molecular groups:** While they have different numbers of antigenic sites, the term "molecular groups" is too vague and does not specifically describe the biochemical basis of blood group specificity. * **Ionic charges:** Surface charge (Zeta potential) is primarily determined by sialic acid on the RBC membrane and does not define the specificity of A1 vs. A2 subtypes. **Clinical Pearls for NEET-PG:** * **Anti-A1 Antibody:** About 1–8% of A2 individuals and 25% of A2B individuals produce naturally occurring Anti-A1. This can cause **ABO discrepancy** during cross-matching. * **Lectins:** *Dolichos biflorus* is the lectin used to identify A1 cells (it agglutinates A1 but not A2). * **A1 vs. A2:** A1 is the most common subtype (80%), while A2 is the second most common (20%).

Question 142: What causes the chloride shift?

A. Generation of bicarbonate ions within red blood cells (Correct Answer)
B. Metabolism of glucose within red blood cells
C. Formation of oxyhemoglobin within red blood cells
D. None of the above

Explanation: ### Explanation The **Chloride Shift (also known as the Hamburger Phenomenon)** is a crucial mechanism for maintaining electrical neutrality within red blood cells (RBCs) during the transport of carbon dioxide. **1. Why Option A is Correct:** When $CO_2$ enters the RBC from tissues, it reacts with water (catalyzed by **Carbonic Anhydrase**) to form carbonic acid ($H_2CO_3$), which dissociates into hydrogen ions ($H^+$) and **bicarbonate ions ($HCO_3^-$)**. As $HCO_3^-$ concentration rises, it diffuses out of the RBC into the plasma along its concentration gradient. To maintain electrical equilibrium, negatively charged **Chloride ions ($Cl^-$)** shift from the plasma into the RBC. Therefore, the generation and subsequent efflux of bicarbonate is the direct trigger for this shift. **2. Why Other Options are Incorrect:** * **Option B:** Glucose metabolism (glycolysis) provides energy (ATP) and 2,3-BPG for the RBC but does not directly drive the rapid ionic exchange required for $CO_2$ transport. * **Option C:** The formation of oxyhemoglobin occurs in the lungs. This actually triggers the **Reverse Chloride Shift**, where $Cl^-$ moves out of the RBC as $HCO_3^-$ moves back in to be converted to $CO_2$ and exhaled. **3. High-Yield Clinical Pearls for NEET-PG:** * **Direction:** In systemic capillaries (tissues), Chloride moves **into** the RBC. In pulmonary capillaries (lungs), Chloride moves **out** of the RBC. * **Water Follows Salt:** As $Cl^-$ enters the RBC at the tissues, it increases the intracellular osmotic pressure, causing water to enter. This results in **venous RBCs having a slightly larger volume (higher MCV)** than arterial RBCs. * **Enzyme:** Carbonic Anhydrase is the fastest known enzyme and is essential for this process. * **Transporter:** The exchange is mediated by the **Band 3 protein** (anion exchanger 1).

Question 143: What is the typical lifespan of a monocyte in the bloodstream?

A. 10 - 20 hours (Correct Answer)
B. 1 - 3 days
C. 1 - 2 weeks
D. 1 - 2 months

Explanation: ### Explanation **1. Why Option A is Correct:** Monocytes are the largest leukocytes in the peripheral blood. After being produced in the bone marrow (monopoiesis), they enter the circulation where they remain for a very short duration. According to standard physiological texts (like Guyton and Ganong), the average transit time or lifespan of a monocyte in the bloodstream is approximately **10 to 20 hours**. After this brief period, they migrate through the capillary walls into the tissues. **2. Analysis of Incorrect Options:** * **Option B (1 - 3 days):** While some sources suggest monocytes can circulate for up to 72 hours in certain inflammatory states, the "typical" physiological lifespan is much shorter (under 24 hours). * **Option C & D (Weeks to Months):** These timeframes refer to the lifespan of **Tissue Macrophages**. Once a monocyte enters the tissue (becoming a Kupffer cell in the liver, alveolar macrophage in the lungs, or microglia in the brain), its lifespan increases dramatically, lasting for months or even years unless destroyed while performing phagocytosis. **3. NEET-PG High-Yield Clinical Pearls:** * **The Monocyte-Macrophage System:** Formerly known as the Reticuloendothelial System (RES). * **Function:** Monocytes are immature cells; their true phagocytic power is only realized once they swell and mature into tissue macrophages. * **Differentiation:** Unlike neutrophils (which are "end cells"), monocytes can divide and replicate within the tissues to maintain their population. * **Diagnostic Significance:** Monocytosis is classically associated with chronic infections like **Tuberculosis, Syphilis, and Subacute Bacterial Endocarditis (SBE)**, as well as protozoal infections like Malaria.

Question 144: Which plasma protein is necessary for the adhesion of platelets to subendothelial fibres?

A. Glycoprotein IIb
B. Von Willebrand factor (Correct Answer)
C. Platelet factor 3
D. Factor X

Explanation: ### Explanation **Correct Answer: B. Von Willebrand factor (vWF)** The process of primary hemostasis begins with **platelet adhesion**. When a blood vessel is injured, subendothelial collagen is exposed. Platelets cannot bind directly to collagen under high-shear conditions (like in arteries). **Von Willebrand factor (vWF)** acts as a molecular bridge: one end binds to the exposed **subendothelial collagen**, and the other end binds to the **Glycoprotein Ib (GpIb)** receptor on the platelet surface. This tethering is essential for the initial attachment of platelets to the site of injury. **Analysis of Incorrect Options:** * **A. Glycoprotein IIb (GpIIb):** This is part of the GpIIb/IIIa complex. It is responsible for **platelet aggregation** (binding platelets to each other via fibrinogen), not adhesion to the subendothelium. * **C. Platelet factor 3 (PF3):** This is a phospholipid lipoprotein on the platelet membrane that provides a surface for the assembly of coagulation factors (the "tenase" and "prothrombinase" complexes). * **D. Factor X:** This is a key enzyme in the common pathway of the coagulation cascade that converts prothrombin to thrombin. It is not involved in the initial adhesion of platelets. **High-Yield Clinical Pearls for NEET-PG:** * **Bernard-Soulier Syndrome:** Deficiency of the **GpIb** receptor (results in defective adhesion). Characterized by giant platelets and thrombocytopenia. * **Glanzmann Thrombasthenia:** Deficiency of the **GpIIb/IIIa** complex (results in defective aggregation). * **Von Willebrand Disease (vWD):** The most common inherited bleeding disorder. It leads to a dual defect: impaired platelet adhesion and a secondary deficiency of **Factor VIII** (vWF stabilizes Factor VIII in circulation). * **Ristocetin Cofactor Assay:** Used to test vWF function; ristocetin induces platelet agglutination only in the presence of vWF.

Question 145: What is the normal reduced hemoglobin level in blood?

A. 3% (Correct Answer)
B. 10%
C. 20%
D. 30%

Explanation: **Explanation:** In a healthy individual, the vast majority of hemoglobin exists as **oxyhemoglobin** (bound to oxygen). However, a small fraction remains in the **deoxygenated state**, known as **reduced hemoglobin**. Under normal physiological conditions, the concentration of reduced hemoglobin in systemic arterial blood is approximately **3%** (or roughly 0.4–0.5 g/dL). **Why Option A is correct:** The oxygen dissociation curve ensures that at a normal arterial $PaO_2$ of 95–100 mmHg, hemoglobin is about 97% saturated. The remaining **3%** represents the reduced hemoglobin. This balance is critical because an increase in reduced hemoglobin leads to clinical manifestations like cyanosis. **Why the other options are incorrect:** * **Option B (10%):** This level is abnormally high for arterial blood. If reduced hemoglobin reaches this percentage, it often indicates mild hypoxia or impaired gas exchange. * **Options C & D (20% and 30%):** These levels represent severe hypoxemia. At these concentrations, the absolute amount of reduced hemoglobin would far exceed the threshold for clinical cyanosis, indicating life-threatening respiratory or circulatory failure. **High-Yield Clinical Pearls for NEET-PG:** * **Cyanosis Threshold:** Central cyanosis becomes clinically apparent only when the absolute concentration of reduced hemoglobin in the capillaries exceeds **5 g/dL**. * **Anemia vs. Polycythemia:** A severely anemic patient may never show cyanosis (even if hypoxic) because they cannot reach the 5 g/dL threshold of reduced Hb. Conversely, polycythemic patients may show cyanosis more easily. * **Methemoglobin:** Do not confuse reduced Hb with methemoglobin (where iron is in the $Fe^{3+}$ state); normal methemoglobin levels are <1%.

Question 146: The extrinsic pathway of clotting is activated by?

A. Release of tissue thromboplastin (Correct Answer)
B. Conversion of fibrinogen into fibrin
C. Formation of prothrombin
D. Release of calcium

Explanation: **Explanation:** The coagulation cascade is divided into the intrinsic and extrinsic pathways, which converge into a common pathway. **Why Option A is Correct:** The **extrinsic pathway** is the body’s rapid response to tissue injury. It is initiated when damaged extravascular tissues release **Tissue Factor (TF)**, also known as **Tissue Thromboplastin (Factor III)**. Once released, Factor III binds with Factor VII in the presence of calcium to form a complex that directly activates Factor X, marking the start of the common pathway. It is called "extrinsic" because the initiating factor (TF) is external to the blood. **Why Other Options are Incorrect:** * **B & C:** The formation of prothrombin and the conversion of fibrinogen to fibrin are steps in the **Common Pathway**. These occur *after* the extrinsic or intrinsic pathways have already been activated. * **D:** Calcium (Factor IV) is a necessary cofactor for almost all steps of the clotting cascade (except the first two steps of the intrinsic pathway), but it is not the *initiator* of the extrinsic pathway. **NEET-PG High-Yield Pearls:** * **Speed:** The extrinsic pathway is much faster than the intrinsic pathway, occurring in seconds. * **Monitoring:** The extrinsic pathway is clinically monitored using **Prothrombin Time (PT)**. * **Key Factor:** Factor VII has the shortest half-life of all clotting factors; therefore, PT is the first to be prolonged in liver disease or Vitamin K deficiency. * **Mnemonic:** "The **Ex**-boyfriend (Extrinsic) is **PT** (Petty) and lives **Outside** (Tissue Factor)."

Question 147: Red cell volume can be measured by injecting tagged red blood cells. All of the following are used to tag red cells, EXCEPT:

A. 51Cr
B. 59Fe
C. 32P
D. 121I (Correct Answer)

Explanation: To measure **Red Cell Volume (RCV)**, the principle of **indicator dilution** is used. This requires a tracer that binds specifically and firmly to erythrocytes. ### Why 121I is the Correct Answer **121I (Radioactive Iodine)** is not used to tag red blood cells. Iodine isotopes (specifically **125I** and **131I**) are used to label **Serum Albumin**. Therefore, radio-iodinated albumin is the gold standard for measuring **Plasma Volume**, not red cell volume. Using an iodine tracer for RCV would yield inaccurate results as it remains in the extracellular fluid/plasma compartment. ### Explanation of Incorrect Options (Used for RCV) * **51Cr (Radioactive Chromium):** This is the **most common** and standard method for measuring RCV. It binds to the beta chain of hemoglobin. * **32P (Radioactive Phosphorus):** This was historically used to label RBCs by incorporating into the cell membrane/metabolism, though it is less common now than 51Cr. * **59Fe (Radioactive Iron):** Iron is a core component of heme. It can be used to tag RBCs, though it is more frequently used in studies of erythropoiesis and iron kinetics. ### Clinical Pearls for NEET-PG * **Total Blood Volume:** Measured using **51Cr-labeled RBCs** (most accurate) or calculated as *Plasma Volume / (1 - Hematocrit)*. * **Plasma Volume tracers:** 125I-Albumin, 131I-Albumin, and **Evans Blue dye** (T-1824). * **Extracellular Fluid (ECF) tracers:** Inulin (Gold standard), Mannitol, Sucrose, and isotopes of Na+, Cl-, or Br-. * **Total Body Water tracers:** Deuterium Oxide ($D_2O$), Tritium Oxide ($T_2O$), and Aminopyrine.

Question 148: Loose fibrin is stabilized in a tight clot by which coagulation factor?

A. Factor X
B. Factor XI
C. Factor XII
D. Factor XIII (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Factor XIII** **Underlying Concept:** The final step of the coagulation cascade involves the conversion of soluble fibrinogen into insoluble fibrin. Initially, thrombin converts fibrinogen into **fibrin monomers**, which aggregate to form a weak, "loose" meshwork (held together by hydrogen bonds). To transform this into a stable, "tight" clot, **Factor XIII (Fibrin Stabilizing Factor)** is required. Factor XIII is a transglutaminase activated by thrombin in the presence of calcium. It creates **covalent cross-links** between the glutamine and lysine residues of adjacent fibrin strands. This cross-linking provides the clot with mechanical strength and resistance to premature lysis by plasmin. **Why Incorrect Options are Wrong:** * **Factor X (Stuart-Prower Factor):** This is the point where the intrinsic and extrinsic pathways converge to form the **Prothrombin Activator complex**. Its role is to convert prothrombin to thrombin, not to stabilize fibrin. * **Factor XI (Plasma Thromboplastin Antecedent):** Part of the intrinsic pathway; it activates Factor IX. Deficiency leads to Hemophilia C. * **Factor XII (Hageman Factor):** This factor initiates the intrinsic pathway upon contact with negatively charged surfaces (collagen/glass). While vital for *in vitro* clotting, its deficiency does not typically cause clinical bleeding. **NEET-PG High-Yield Pearls:** * **Factor XIII Deficiency:** Characterized by delayed bleeding (clot forms but breaks down) and poor wound healing. A classic presentation is **delayed umbilical cord bleeding** in neonates. * **Screening Test:** Routine PT and APTT are **normal** in Factor XIII deficiency. The diagnostic test is the **Urea Solubility Test** (the clot dissolves in 5M urea or 1% monochloroacetic acid if Factor XIII is absent). * **Vitamin K Dependent Factors:** II, VII, IX, X (Factor XIII is NOT Vitamin K dependent).

Question 149: Progenitor hematopoietic stem cells originate in which of the following?

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

Explanation: ### Explanation **Correct Option: A. Bone Marrow** In adults, the **bone marrow** is the primary site of hematopoiesis. All blood cells, including red blood cells, white blood cells, and platelets, arise from a common multipotent cell known as the **Pluripotent Hematopoietic Stem Cell (PHSC)**. These PHSCs give rise to **Progenitor cells** (Committed Stem Cells), such as the Colony Forming Unit-Erythrocyte (CFU-E) and CFU-Granulocyte/Monocyte (CFU-GM). While hematopoiesis occurs in the yolk sac, liver, and spleen during fetal life, the bone marrow takes over as the sole physiological source of progenitor cells from the third trimester onwards. **Analysis of Incorrect Options:** * **B. Thymus:** This is a primary lymphoid organ, but it does not produce progenitor cells. Instead, immature T-lymphocyte precursors migrate from the bone marrow to the thymus to undergo maturation and "education." * **C. Lymph Nodes:** These are secondary lymphoid organs. They are sites where mature lymphocytes encounter antigens and undergo activation and proliferation, but they do not originate hematopoietic stem cells. * **D. Spleen:** While the spleen is a major site of **extramedullary hematopoiesis** during fetal development (months 3–7) or in pathological states (e.g., Myelofibrosis), it is not the standard site of origin for progenitor cells in a healthy postnatal individual. **High-Yield Clinical Pearls for NEET-PG:** * **Sequence of Hematopoiesis:** Yolk sac (3rd week) → Liver (6th week, primary site until mid-fetal life) → Spleen (3rd to 7th month) → Bone Marrow (starts at 4th month, becomes primary by birth). * **Stem Cell Marker:** Hematopoietic stem cells are characterized by the expression of the **CD34+** surface marker, which is used clinically for stem cell harvesting in transplants. * **Red vs. Yellow Marrow:** In children, almost all marrow is "red" (active). In adults, active hematopoiesis is limited to the axial skeleton (skull, vertebrae, ribs, pelvis) and the proximal ends of the femur and humerus.

Question 150: How do platelets aid in clot stabilization?

A. Release of factor 8
B. Release of calcium (Correct Answer)
C. Release of thrombomodulin
D. Activation of thrombasthenin

Explanation: **Explanation:** Platelets play a pivotal role in the coagulation cascade beyond just forming a primary plug. The correct answer is **Release of calcium** because calcium (Factor IV) is an essential cofactor for almost every step of the clotting cascade. 1. **Why Calcium is Correct:** Platelets contain **dense granules** (delta granules) that store and release high concentrations of ionized calcium. Calcium acts as a "bridge" between clotting factors and the phospholipid surface of activated platelets. It is specifically required for the activation of Factor X and the conversion of prothrombin to thrombin. Without calcium, the secondary hemostasis process cannot stabilize the initial platelet plug into a firm fibrin clot. 2. **Why Other Options are Incorrect:** * **Factor VIII:** This is primarily synthesized in the liver and endothelial cells (associated with von Willebrand factor), not released by platelets. * **Thrombomodulin:** This is an endothelial cell receptor that binds thrombin to activate Protein C (an anticoagulant). It inhibits clotting rather than stabilizing it. * **Thrombasthenin:** While platelets do contain this contractile protein, its role is in **clot retraction** (shrinking the clot) rather than the biochemical stabilization of the fibrin meshwork itself. **High-Yield NEET-PG Pearls:** * **Platelet Granules:** Remember the mnemonic **"SAC"** for Dense granules: **S**erotonin, **A**DP/ATP, and **C**alcium. * **Alpha Granules:** Contain vWF, Fibrinogen, and Platelet-Derived Growth Factor (PDGF). * **Chelating Agents:** Citrate and EDTA prevent blood clotting in vitro by sequestering calcium ions. * **Glanzmann Thrombasthenia:** A deficiency of GpIIb/IIIa, leading to defective platelet aggregation.

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Composition and Functions of Blood

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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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