Blood and Immunity — MCQs

Blood and Immunity Indian Medical PG Practice Questions and MCQs

Question 321: Erythrocyte Sedimentation Rate (ESR) depends on which of the following factors?

A. Viscosity
B. Fibrinogen
C. Rouleaux formation
D. All of the above (Correct Answer)

Explanation: **Explanation:** The Erythrocyte Sedimentation Rate (ESR) measures the rate at which red blood cells (RBCs) sink to the bottom of a vertical tube of anticoagulated blood. This process is governed by the balance between pro-sedimentation factors (gravity) and anti-sedimentation factors (buoyancy and electrostatic repulsion). **Why "All of the above" is correct:** 1. **Fibrinogen:** This is the most important factor. Fibrinogen is an asymmetrical, positively charged acute-phase reactant. It neutralizes the negative surface charge (Zeta potential) of RBCs, allowing them to clump together. 2. **Rouleaux Formation:** When the Zeta potential is neutralized by plasma proteins like fibrinogen or globulins, RBCs stack like coins. These "Rouleaux" have a lower surface-area-to-volume ratio than individual cells, causing them to sediment much faster due to decreased upward frictional resistance. 3. **Viscosity:** ESR is inversely proportional to plasma viscosity. According to Stokes' Law, increased viscosity of the medium (plasma) provides more resistance to the falling particles, thereby slowing down the sedimentation rate. **Clinical Pearls for NEET-PG:** * **Westergren Method:** The gold standard for measuring ESR (uses a 200mm tube). * **Anemia vs. Polycythemia:** Anemia increases ESR (fewer RBCs to collide, allowing faster sinking), while polycythemia decreases ESR (increased viscosity and crowding). * **Sickle Cell Anemia:** Characterized by a very low ESR because the abnormally shaped cells cannot form Rouleaux. * **Highest ESR values:** Typically seen in Multiple Myeloma, Temporal Arteritis, and Polymyalgia Rheumatica. * **Albumin:** Unlike fibrinogen, albumin is a small, symmetrical molecule that actually *decreases* ESR by maintaining the Zeta potential.

Question 322: The universal blood recipient group is:

A. O -Ve
B. O +Ve
C. AB -Ve
D. AB +Ve (Correct Answer)

Explanation: **Explanation:** The concept of blood compatibility is determined by the presence or absence of specific antigens on the surface of Red Blood Cells (RBCs) and corresponding antibodies in the plasma. **Why AB +Ve is the Universal Recipient:** A person with **AB +Ve** blood has **A, B, and Rh (D) antigens** on their RBCs. Consequently, their plasma contains **no antibodies** against A, B, or Rh antigens. Because they lack these antibodies, they can receive blood from any group (A, B, AB, or O, whether Rh positive or negative) without triggering a life-threatening transfusion reaction (hemolysis). **Analysis of Incorrect Options:** * **O -Ve:** This is the **Universal Donor**. These RBCs lack A, B, and Rh antigens, meaning they can be given to patients of any blood type. However, O -Ve individuals have anti-A and anti-B antibodies in their plasma, so they can only receive O -Ve blood. * **O +Ve:** This is the most common blood type. While it can be donated to any Rh+ recipient, O +Ve individuals have anti-A and anti-B antibodies, limiting what they can receive. * **AB -Ve:** While these individuals lack anti-A and anti-B antibodies, they do not have the Rh antigen. If they receive Rh+ blood, they may develop anti-D antibodies, leading to transfusion reactions in the future. **High-Yield Clinical Pearls for NEET-PG:** * **Landsteiner’s Law:** States that if an antigen is present on RBCs, the corresponding antibody must be absent from the plasma (and vice versa). * **Emergency Transfusion:** In an unknown emergency, **O -Ve** packed RBCs are used. * **Plasma vs. Whole Blood:** While AB +Ve is the universal recipient for *cells*, **AB is the universal donor for *plasma*** because it contains no antibodies. * **Bombay Blood Group:** Lacks the H-antigen; these individuals can only receive blood from another Bombay group individual.

Question 323: Which of the following is the earliest event in the clotting cascade?

A. Binding of platelets to red blood cells
B. Binding of vitamin K to endothelial cell surfaces
C. Release of tissue factor by damaged vessels (Correct Answer)
D. Secretion of von Willebrand factor by platelets

Explanation: **Explanation:** The clotting cascade is initiated via two pathways: the **Intrinsic** and the **Extrinsic** pathway. The **Extrinsic pathway** is the primary physiological trigger for coagulation in vivo and is initiated by the **release of Tissue Factor (Factor III)** from damaged subendothelial cells and fibroblasts. Upon vascular injury, Tissue Factor is exposed to the blood, where it binds to Factor VIIa to form the "extrinsic tenase" complex, marking the earliest biochemical step in the cascade. **Analysis of Incorrect Options:** * **Option A:** Platelets do not typically bind to red blood cells as a primary step in clotting; they bind to the subendothelial collagen and von Willebrand factor (vWF). * **Option B:** Vitamin K is a fat-soluble cofactor required for the hepatic synthesis (gamma-carboxylation) of Factors II, VII, IX, and X. It does not bind to endothelial surfaces to initiate the cascade. * **Option D:** While vWF is crucial for platelet adhesion, it is primarily secreted by **endothelial cells** (stored in Weibel-Palade bodies) and is already present in the plasma. Platelet secretion (alpha granules) occurs *after* initial activation, not as the very first event. **High-Yield NEET-PG Pearls:** * **Rate-limiting step:** The activation of Factor X to Xa (the start of the Common Pathway). * **Vitamin K dependent factors:** II, VII, IX, X, Protein C, and Protein S. * **Extrinsic Pathway Marker:** Prothrombin Time (PT). * **Intrinsic Pathway Marker:** Activated Partial Thromboplastin Time (aPTT). * **Calcium (Factor IV):** Required for almost every step of the cascade except the first two steps of the intrinsic pathway.

Question 324: Which of the following cell types give rise to macrophages?

A. Neutrophils
B. Monocytes (Correct Answer)
C. Lymphocytes
D. Reticuloendothelial cells

Explanation: ### Explanation The correct answer is **B. Monocytes**. **1. Why Monocytes are correct:** Monocytes and macrophages are part of the **Mononuclear Phagocyte System (MPS)**. Monocytes are produced in the bone marrow and released into the peripheral blood, where they circulate for approximately 10–20 hours. Once they migrate out of the capillaries into various tissues, they undergo structural and functional changes—increasing in size and lysosomal content—to differentiate into **macrophages**. Macrophages can survive for months or even years in the tissues, acting as the primary professional phagocytes. **2. Why other options are incorrect:** * **Neutrophils (A):** These are granulocytes and the first responders to acute inflammation. They are "end cells" that do not differentiate into other cell types; they typically die after phagocytosis (forming pus). * **Lymphocytes (C):** These are part of the adaptive immune system. B-lymphocytes differentiate into plasma cells, and T-lymphocytes differentiate into effector T-cells. They do not give rise to the myeloid-derived macrophage lineage. * **Reticuloendothelial cells (D):** This is an older term for the system of phagocytic cells (now called MPS). While macrophages are *part* of this system, the system itself is a functional classification, not a precursor cell. **3. High-Yield Clinical Pearls for NEET-PG:** * **Tissue-Specific Macrophages:** Know these names for matching questions: * Liver: **Kupffer cells** * CNS: **Microglia** * Lungs: **Alveolar macrophages (Dust cells)** * Skin: **Langerhans cells** * Bone: **Osteoclasts** * **Cytokine Link:** **Monocyte Chemoattractant Protein-1 (MCP-1)** is the key chemokine that recruits monocytes to sites of inflammation. * **Function:** Beyond phagocytosis, macrophages are crucial **Antigen-Presenting Cells (APCs)** that present antigens to T-cells via MHC II molecules.

Question 325: Factor Xa is necessary for the conversion of prothrombin to thrombin in which pathway?

A. Only in the extrinsic pathway
B. Only in the intrinsic pathway
C. As part of both extrinsic and intrinsic pathways (Correct Answer)
D. Only if the normal blood clotting cascade is inhibited

Explanation: ### Explanation The coagulation cascade is divided into the **Intrinsic**, **Extrinsic**, and **Common pathways**. The correct answer is **C** because Factor Xa is the central protease of the **Common Pathway**, where both the intrinsic and extrinsic pathways converge. **1. Why Option C is Correct:** The conversion of prothrombin (Factor II) to thrombin (Factor IIa) is the hallmark of the Common Pathway. This process is catalyzed by the **Prothrombinase Complex**, which consists of **Factor Xa** (the enzyme), Factor Va (the cofactor), Calcium ions, and phospholipids. Since both the Intrinsic pathway (via the Tenase complex) and the Extrinsic pathway (via Tissue Factor-VIIa) aim to activate Factor X into Factor Xa, Factor Xa is essential for thrombin formation regardless of how the cascade was initiated. **2. Why Other Options are Wrong:** * **Options A & B:** These are incorrect because they are too restrictive. While the pathways start differently (Intrinsic via Factor XII; Extrinsic via Tissue Factor), they are not independent silos. They both terminate by activating Factor X. * **Option D:** This is factually incorrect. Factor Xa is a physiological requirement for normal hemostasis, not a compensatory mechanism for inhibition. **3. NEET-PG Clinical Pearls & High-Yield Facts:** * **The Prothrombinase Complex:** Remember the mnemonic **"5, 10, 2, 1"** (Factors V, X, II, I) to represent the sequence of the Common Pathway. * **Rate-Limiting Step:** The activation of Factor X is often considered the rate-limiting step of coagulation. * **Pharmacology Link:** **Rivaroxaban** and **Apixaban** are Direct Factor Xa inhibitors (DOACs) that block the common pathway, preventing thrombin generation. * **Lab Correlation:** The **PT (Prothrombin Time)** measures the Extrinsic and Common pathways, while **aPTT** measures the Intrinsic and Common pathways. Both will be prolonged if Factor X is deficient.

Question 326: What is the best anticoagulant for the osmotic fragility test?

A. Heparin (Correct Answer)
B. EDTA
C. Trisodium citrate
D. Potassium oxalate

Explanation: **Explanation:** The **Osmotic Fragility Test (OFT)** measures the resistance of red blood cells (RBCs) to hemolysis when exposed to varying concentrations of hypotonic saline. The goal is to determine the surface area-to-volume ratio of the RBCs. **Why Heparin is the Correct Answer:** Heparin is the anticoagulant of choice because it **does not alter the size or shape of the RBCs**. It works by activating antithrombin III, which inhibits thrombin, without interfering with the electrolyte balance or the osmotic pressure of the plasma. For an accurate OFT, the RBCs must remain in their native physiological state; any shrinkage or swelling caused by the anticoagulant would yield a false result. **Why Other Options are Incorrect:** * **EDTA (Ethylenediaminetetraacetic acid):** While excellent for routine CBCs, EDTA is a chelating agent that removes calcium. This process can alter the membrane permeability and slightly change the salt concentration, potentially affecting the osmotic balance. * **Trisodium Citrate:** This is a liquid anticoagulant that causes a **dilutional effect**. It also alters the ionic strength of the plasma, which interferes with the precise saline gradients required for the OFT. * **Potassium Oxalate:** Oxalates work by precipitating calcium. This significantly alters the osmotic pressure of the plasma, causing water to shift out of the RBCs, leading to **cell shrinkage (crenation)**, which invalidates the test. **High-Yield Clinical Pearls for NEET-PG:** * **Increased Osmotic Fragility (Cells burst easily):** Seen in **Hereditary Spherocytosis** (due to low surface area-to-volume ratio). * **Decreased Osmotic Fragility (Cells are resistant):** Seen in **Thalassemia**, Sickle Cell Anemia, and Iron Deficiency Anemia (due to high surface area-to-volume ratio/target cells). * **Incubated OFT:** Sensitivity for Hereditary Spherocytosis increases if the blood is incubated at 37°C for 24 hours before testing.

Question 327: Which factor activates preaccelerin to Vila?

A. Tissue factor (Correct Answer)
B. Christmas factor
C. Fibrinogen
D. Prothrombin

Explanation: **Explanation:** The question refers to the activation of **Factor VII** (proconvertin) into **Factor VIIa**. Note: There is a slight nomenclature nuance in the question; **Preaccelerin** is technically the name for **Factor V**, while **Factor VII** is Proconvertin. However, in the context of the extrinsic pathway initiation, the factor activated by **Tissue Factor (TF)** is Factor VII. 1. **Why Tissue Factor is correct:** The **Extrinsic Pathway** of coagulation is initiated when subendothelial **Tissue Factor (Factor III)** is exposed to the blood due to vascular injury. Tissue Factor acts as a high-affinity receptor and a mandatory cofactor for Factor VII. Upon binding to TF in the presence of calcium ions, Factor VII is rapidly converted into its active form, **Factor VIIa**. This TF-VIIa complex then activates Factors X and IX. 2. **Analysis of Incorrect Options:** * **Christmas Factor (Factor IX):** This is a component of the Intrinsic Pathway. It is activated by Factor XIa and, in turn, activates Factor X (with Factor VIIIa). * **Fibrinogen (Factor I):** This is the soluble precursor that is converted into insoluble **Fibrin** by Thrombin at the final stage of the common pathway. * **Prothrombin (Factor II):** This is the precursor to **Thrombin**. It is activated by the prothrombinase complex (Xa, Va, Ca²⁺, and phospholipids). **High-Yield Clinical Pearls for NEET-PG:** * **Factor VII** has the **shortest half-life** (approx. 4–6 hours) among all clotting factors. Therefore, it is the first factor to decrease in Vitamin K deficiency or liver disease, making **Prothrombin Time (PT)** the earliest indicator of liver dysfunction. * **Factor V (Preaccelerin)** is a cofactor for Factor Xa; its deficiency or mutation (Factor V Leiden) is a common cause of inherited thrombophilia. * The **Extrinsic Pathway** is monitored by **PT/INR**, while the **Intrinsic Pathway** is monitored by **aPTT**.

Question 328: Which of the following cells forms Major Basic Protein?

A. Lymphocyte
B. Basophil
C. Neutrophil
D. Eosinophil (Correct Answer)

Explanation: **Explanation:** **Correct Option: D (Eosinophil)** Eosinophils are specialized granulocytes primarily involved in defending the body against parasitic infections and modulating allergic reactions. The characteristic acidophilic (pink) granules of eosinophils contain several cytotoxic proteins, the most abundant being **Major Basic Protein (MBP)**. MBP is highly toxic to helminths (parasites) as it disrupts their cell membranes. It also triggers the release of histamine from mast cells and basophils, contributing to the inflammatory response in conditions like bronchial asthma. **Why other options are incorrect:** * **A. Lymphocytes:** These are agranulocytes involved in adaptive immunity (B-cells and T-cells). They produce antibodies or cytokines, not MBP. * **B. Basophils:** While they share some functional similarities with eosinophils in allergies, their granules primarily contain **histamine, heparin, and serotonin**. * **C. Neutrophils:** These are the primary cells for acute bacterial defense. Their granules contain enzymes like **myeloperoxidase (MPO)**, lysozyme, and alkaline phosphatase, but not MBP. **High-Yield Clinical Pearls for NEET-PG:** * **Eosinophil Granule Contents:** Besides MBP, they contain Eosinophil Cationic Protein (ECP), Eosinophil Peroxidase (EPO), and Eosinophil-Derived Neurotoxin (EDN). * **Charcot-Leyden Crystals:** Found in the sputum of asthmatics (Curschmann spirals), these crystals are formed from the breakdown of eosinophil membrane proteins (**Galectin-10**). * **Eosinophilia:** Classically seen in **NAACP**: **N**eoplasia, **A**llergy/Asthma, **A**ddison’s disease, **C**onnective tissue disorders, and **P**arasitic infections.

Question 329: Which component is contained within Red Blood Cells?

A. Iron (Correct Answer)
B. Folic acid
C. Vitamin C
D. Biotin

Explanation: **Explanation:** The correct answer is **Iron (Option A)**. Red Blood Cells (RBCs) are primarily designed for the transport of oxygen and carbon dioxide. This function is facilitated by **Hemoglobin**, a complex protein that makes up about 95% of the RBC's dry weight. Each hemoglobin molecule consists of four heme groups, and at the center of each heme group lies an **Iron atom (in the ferrous state, Fe²⁺)**. This iron atom binds reversibly with oxygen, allowing RBCs to carry oxygen from the lungs to the tissues. **Why other options are incorrect:** * **Folic acid (B9):** While essential for erythropoiesis (RBC production) as a co-factor for DNA synthesis in the bone marrow, it is not a structural component *contained within* the mature RBC. * **Vitamin C:** This is a water-soluble antioxidant found in the plasma. While it aids in the absorption of iron from the gut, it is not a constituent of the RBC itself. * **Biotin (B7):** This is a co-enzyme involved in carboxylation reactions in various metabolic pathways, but it plays no structural or functional role inside the red blood cell. **High-Yield Clinical Pearls for NEET-PG:** * **State of Iron:** Iron must be in the **Ferrous (Fe²⁺)** state to bind oxygen. If oxidized to the **Ferric (Fe³⁺)** state, it forms **Methemoglobin**, which cannot bind oxygen, leading to cyanosis. * **Iron Storage:** While iron is *functional* in RBCs, it is *stored* in the body as **Ferritin** (soluble) and **Hemosiderin** (insoluble) in the liver, spleen, and bone marrow. * **RBC Metabolism:** Mature RBCs lack a nucleus and mitochondria; they rely solely on **anaerobic glycolysis** (Embden-Meyerhof pathway) for energy.

Question 330: Fetal hemoglobin shows higher oxygen saturation compared to adult hemoglobin because?

A. The affinity of fetal hemoglobin for binding to 2,3-DPG is different from adult hemoglobin. (Correct Answer)
B. There is a right shift in the oxygen dissociation curve.
C. The environment is acidic.
D. There is cooperative binding of oxygen.

Explanation: **Explanation:** The correct answer is **A**. The primary reason fetal hemoglobin (HbF) has a higher affinity for oxygen than adult hemoglobin (HbA) lies in its structural composition. HbA consists of two alpha and two beta chains ($\alpha_2\beta_2$), while HbF consists of two alpha and two gamma chains ($\alpha_2\gamma_2$). **The Role of 2,3-DPG:** 2,3-Diphosphoglycerate (2,3-DPG) is an allosteric effector that binds to the beta chains of HbA, stabilizing the "T" (tense) state and promoting oxygen unloading. However, the gamma chains in HbF have a lower affinity for 2,3-DPG. Because HbF does not bind 2,3-DPG effectively, it remains in the "R" (relaxed) state, allowing it to bind oxygen more tightly. This creates a partial pressure gradient that facilitates the transfer of oxygen from maternal blood to fetal blood across the placenta. **Analysis of Incorrect Options:** * **Option B:** HbF causes a **left shift** in the oxygen dissociation curve (ODC), indicating higher affinity. A right shift indicates lower affinity and increased unloading. * **Option C:** An acidic environment (Bohr effect) causes a right shift in the ODC, decreasing hemoglobin's affinity for oxygen. * **Option D:** Cooperative binding is a property of both HbA and HbF; it explains the sigmoidal shape of the curve but not the *difference* in affinity between the two. **High-Yield Pearls for NEET-PG:** * **P50 Value:** The P50 (partial pressure at 50% saturation) for HbF is ~19 mmHg, compared to ~27 mmHg for HbA. A lower P50 signifies higher affinity. * **Gamma Chain Difference:** The substitution of **Serine** for Histidine at the 143rd position in the gamma chain reduces the positive charge in the central pocket, weakening the binding of the negatively charged 2,3-DPG. * **Switch:** HbF is the primary hemoglobin from 10–12 weeks of gestation until roughly 6 months post-delivery, when HbA becomes dominant.

Practice by Chapter

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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