Blood and Immunity — MCQs

Blood and Immunity Indian Medical PG Practice Questions and MCQs

Question 121: Which of the following chemokines binds to the CCR3 receptor?

A. IL-8
B. Eotaxin-2 (Correct Answer)
C. CCL3
D. Fractalkine

Explanation: **Explanation:** Chemokines are small signaling proteins that direct the migration of leukocytes (chemotaxis). They are classified into four subfamilies (CXC, CC, C, and CX3C) based on the arrangement of conserved cysteine residues. **1. Why Eotaxin-2 is Correct:** **Eotaxin-2 (CCL24)** is a potent chemoattractant specifically for eosinophils. It binds primarily to the **CCR3 receptor**, which is highly expressed on the surface of eosinophils, basophils, and Th2 cells. This interaction is crucial in the recruitment of these cells to sites of allergic inflammation, such as in bronchial asthma or parasitic infections. **2. Analysis of Incorrect Options:** * **Option A: IL-8 (CXCL8):** This is the prototypical CXC chemokine. It binds to **CXCR1 and CXCR2** receptors and is the primary recruiter for **neutrophils**. * **Option C: CCL3 (MIP-1α):** Macrophage Inflammatory Protein-1α binds to **CCR1 and CCR5**. It is involved in the recruitment of monocytes and T-lymphocytes. * **Option D: Fractalkine (CX3CL1):** This is the only member of the CX3C class. It exists in both membrane-bound and soluble forms and binds to the **CX3CR1** receptor. **High-Yield NEET-PG Pearls:** * **CCR5:** Significant as a co-receptor for **HIV-1** entry into macrophages. Individuals with a CCR5-Δ32 mutation show resistance to HIV. * **CXCR4:** Another major co-receptor for HIV entry (T-cell tropic strains). * **Duffy Antigen (DARC):** A chemokine receptor on RBCs that acts as a "sink" for excess chemokines and serves as the attachment site for *Plasmodium vivax*. * **Eosinophil Recruitment:** Remember the "3 E's": **E**otaxin, **E**osinophil, and CCR**3**.

Question 122: Which of the following is not a component of secondary hemostasis?

A. Thrombin activation
B. Fibrin polymerization
C. Formation of coagulation factor complexes
D. Platelet degranulation (Correct Answer)

Explanation: Hemostasis is the physiological process that stops bleeding at the site of vascular injury. It occurs in two distinct but overlapping phases: **1. Why "Platelet degranulation" is the correct answer:** Platelet degranulation is a hallmark of **Primary Hemostasis**. When blood vessels are injured, platelets adhere to the subendothelial collagen (via von Willebrand factor). This triggers activation and degranulation, where platelets release ADP and Thromboxane A2 to recruit more platelets and form a fragile **platelet plug**. Since the question asks for a component that is *not* part of secondary hemostasis, this is the correct choice. **2. Why the other options are incorrect:** Secondary hemostasis involves the **coagulation cascade**, which aims to stabilize the initial platelet plug by forming a cross-linked fibrin mesh. * **Formation of coagulation factor complexes (Option C):** This is the core of the cascade (e.g., the Tenase and Prothrombinase complexes). * **Thrombin activation (Option A):** This is the "explosive" step where prothrombin is converted to thrombin (Factor IIa). * **Fibrin polymerization (Option B):** Thrombin converts soluble fibrinogen into insoluble fibrin monomers, which then polymerize to form the stable clot. **Clinical Pearls for NEET-PG:** * **Primary Hemostasis defects:** Present with mucosal bleeding, petechiae, and prolonged **Bleeding Time (BT)** (e.g., von Willebrand Disease, ITP). * **Secondary Hemostasis defects:** Present with deep tissue hematomas and hemarthrosis (joint bleeding) and prolonged **PT/aPTT** (e.g., Hemophilia). * **Vitamin K-dependent factors:** II, VII, IX, X, and Proteins C and S. * **Rate-limiting step:** The activation of Factor X is the convergence point of the intrinsic and extrinsic pathways.

Question 123: Which coagulation pathway is activated upon contact with a glass test tube?

A. Intrinsic pathway (Correct Answer)
B. Extrinsic pathway
C. Both pathways
D. Neither pathway

Explanation: ### Explanation **Correct Option: A. Intrinsic pathway** The **Intrinsic pathway** is initiated when blood comes into contact with a **negatively charged surface**, such as the silica in a glass test tube, collagen in a damaged vessel wall, or basement membranes. This process is known as **contact activation**. * **Mechanism:** Contact with glass activates **Factor XII (Hageman factor)**. Once activated (XIIa), it triggers a cascade involving Factor XI, Factor IX, and Factor VIII, eventually leading to the activation of Factor X in the common pathway. * **Key Fact:** This pathway is called "intrinsic" because all the components required for it to function are found within the circulating blood itself. **Why other options are wrong:** * **B. Extrinsic pathway:** This pathway is triggered by **Tissue Factor (Factor III/Thromboplastin)**, which is released from damaged tissues *outside* the blood vessels. It is not activated by simple contact with glass. * **C & D:** Since the activation mechanism (surface contact) specifically targets Factor XII, only the intrinsic pathway is initially triggered. --- ### High-Yield Clinical Pearls for NEET-PG * **Laboratory Test:** The **aPTT (activated Partial Thromboplastin Time)** measures the integrity of the Intrinsic and Common pathways. * **Factor XII Deficiency:** Interestingly, patients with a deficiency in Factor XII (Hageman factor) show a prolonged aPTT in the lab but **do not have clinical bleeding disorders** in vivo. * **The "Glass" Connection:** In modern labs, plastic tubes are often coated with silica (glass particles) specifically to activate the intrinsic pathway and speed up clot formation for serum collection. * **Sequence of Activation:** Remember the "Countdown" for the Intrinsic pathway: **12 → 11 → 9 → 8** (skipping 10, which is the start of the Common pathway).

Question 124: Which of the following facilitates the entry of iron?

A. Hepcidin
B. DMT-2
C. DMT-1 (Correct Answer)
D. Ferroportin

Explanation: **Explanation:** Iron absorption is a tightly regulated process occurring primarily in the duodenum and proximal jejunum. The correct answer is **DMT-1 (Divalent Metal Transporter 1)**. 1. **Why DMT-1 is correct:** Iron in the diet exists as ferric ($Fe^{3+}$) or ferrous ($Fe^{2+}$) iron. After ferric iron is reduced to the ferrous state by duodenal cytochrome B (DcytB), **DMT-1** acts as the primary apical transporter that facilitates the entry of $Fe^{2+}$ from the intestinal lumen into the enterocyte. It is a symporter that transports iron along with protons ($H^+$). 2. **Why other options are incorrect:** * **Hepcidin:** This is the master regulator of iron homeostasis produced by the liver. It **inhibits** iron entry into the plasma by causing the degradation of ferroportin. High hepcidin levels decrease iron absorption. * **DMT-2:** This is a distractor; it is not a recognized major transporter in human iron metabolism. * **Ferroportin:** While it facilitates iron movement, it is the **basolateral** exporter that moves iron *out* of the enterocyte into the blood, rather than facilitating its initial entry into the cell. **High-Yield Clinical Pearls for NEET-PG:** * **Heme Iron:** Absorbed more efficiently than non-heme iron via the **HCP-1** (Heme Carrier Protein 1). * **Ferroportin:** The only known iron exporter in mammals; its deficiency or inhibition leads to iron-restricted erythropoiesis. * **Hepcidin Regulation:** Levels increase during inflammation (via IL-6), leading to "Anemia of Chronic Disease" due to iron sequestration. * **Vitamin C:** Enhances iron absorption by maintaining iron in the soluble ferrous ($Fe^{2+}$) state.

Question 125: Up to 6 weeks of gestation, in which structure does erythropoiesis primarily occur?

A. Yolk sac (Correct Answer)
B. Bone marrow
C. Spleen
D. Liver

Explanation: **Explanation:** Erythropoiesis (the formation of red blood cells) occurs in distinct stages during intrauterine life, transitioning through three specific phases: **Mesoblastic, Hepatic, and Myeloid.** **1. Why A is Correct:** The **Mesoblastic stage** is the earliest phase of erythropoiesis. It begins around the 3rd week of gestation within the **yolk sac** (specifically in the blood islands). This remains the primary site of RBC production until approximately the **6th week** of gestation. These early cells are large, nucleated macroblasts. **2. Why the other options are incorrect:** * **D. Liver:** This marks the **Hepatic stage**. The liver takes over as the primary site of erythropoiesis from the 6th week until the mid-trimester (peak at 3–5 months). While it starts at 6 weeks, the yolk sac is the answer for the period *up to* 6 weeks. * **C. Spleen:** The spleen contributes to erythropoiesis during the second trimester (months 3 to 6) but is never the primary site compared to the liver. * **B. Bone Marrow:** This is the **Myeloid stage**. The bone marrow begins producing blood cells around the 4th to 5th month and becomes the dominant site from the 7th month onwards and throughout postnatal life. **NEET-PG High-Yield Pearls:** * **Sequence of sites:** Yolk Sac → Liver → Spleen → Bone Marrow. * **Hemoglobin types:** Yolk sac produces embryonic hemoglobins (Gower I, Gower II, and Portland). Fetal hemoglobin (HbF) is primarily produced in the liver. * **Post-birth:** In adults, erythropoiesis is restricted to the membranous bones (vertebrae, sternum, ribs, and ilia) after age 20, as the shafts of long bones undergo fat replacement.

Question 126: What are the two embryonic hemoglobins?

A. Fetal hemoglobin and Gower hemoglobin
B. Fetal hemoglobin and Poland hemoglobin
C. Poland hemoglobin and HbA2
D. Gower hemoglobin and Poland haemoglobin (Correct Answer)

Explanation: **Explanation:** In human development, hemoglobin synthesis occurs in three distinct stages: embryonic, fetal, and adult. The embryonic stage occurs primarily in the **yolk sac** during the first 8–10 weeks of gestation. **1. Why the Correct Answer is Right:** The embryonic hemoglobins are characterized by the presence of epsilon (ε) and zeta (ζ) globin chains. The three primary embryonic hemoglobins are: * **Hb Gower 1** (ζ₂ε₂) * **Hb Gower 2** (α₂ε₂) * **Hb Portland** (ζ₂γ₂) Option D correctly identifies **Gower** and **Portland** (often referred to in older texts or specific contexts as Poland/Portland) as the embryonic variants. These are replaced by Fetal Hemoglobin (HbF) as erythropoiesis shifts to the liver and spleen. **2. Analysis of Incorrect Options:** * **Option A & B:** **Fetal Hemoglobin (HbF - α₂γ₂)** is the predominant hemoglobin from the 8th week of gestation until birth. While it exists during the fetal period, it is distinct from the initial "embryonic" hemoglobins produced in the yolk sac. * **Option C:** **HbA2 (α₂δ₂)** is a minor component of normal **adult** hemoglobin (approx. 2–3%). It is not present during embryonic development. **3. High-Yield Facts for NEET-PG:** * **Sites of Erythropoiesis:** Yolk sac (Mesoblastic stage) → Liver/Spleen (Hepatic stage) → Bone Marrow (Myeloid stage). * **HbF Structure:** α₂γ₂. It has a **higher affinity for oxygen** than adult hemoglobin (HbA) because it binds poorly to 2,3-BPG, ensuring oxygen transfer from mother to fetus. * **Adult Hemoglobin:** HbA (α₂β₂) constitutes ~97% of hemoglobin in adults. * **Switching:** The "Gamma to Beta" switch occurs around the time of birth.

Question 127: Where does hematopoiesis first start?

A. Liver
B. Yolk sac (Correct Answer)
C. Bone marrow
D. Spleen

Explanation: **Explanation:** Hematopoiesis (the formation of blood cells) is a dynamic process that shifts locations throughout intrauterine development. This is a high-yield topic for NEET-PG, often tested via the "Timeline of Hematopoiesis." **1. Why Yolk Sac is Correct:** Hematopoiesis begins in the **Mesoderm of the Yolk Sac** during the **3rd week** of gestation. This is known as the **Mesoblastic stage**. These early cells are primarily nucleated red blood cells containing embryonic hemoglobins (Gower 1, Gower 2, and Portland). **2. Analysis of Incorrect Options:** * **Liver (Option A):** The liver becomes the primary site of hematopoiesis during the **Hepatic stage**, starting around the **6th week** and peaking at the 3rd–4th month. It remains active until shortly before birth. * **Bone Marrow (Option B):** This is the **Myeloid stage**. Hematopoiesis begins in the bone marrow around the **4th–5th month** (20th week) and becomes the definitive site for blood cell production after birth. * **Spleen (Option D):** The spleen contributes to hematopoiesis primarily between the **3rd and 6th months** of gestation, but it is never the *first* site. **Clinical Pearls for NEET-PG:** * **Sequence mnemonic:** **"Young Liver Synthesizes Blood"** (Yolk sac → Liver → Spleen → Bone marrow). * **Adult sites:** In adults, hematopoiesis is restricted to the **axial skeleton** (vertebrae, sternum, ribs, pelvis) and proximal ends of the femur/humerus. * **Extramedullary Hematopoiesis:** In certain pathological states (e.g., Myelofibrosis or Thalassemia), the liver and spleen can resume blood cell production, leading to hepatosplenomegaly.

Question 128: Erythropoietin is secreted by?

A. Pituitary
B. Lung
C. Spleen
D. Kidney (Correct Answer)

Explanation: **Explanation:** **Erythropoietin (EPO)** is a glycoprotein hormone that serves as the primary regulator of erythropoiesis (red blood cell production). 1. **Why Kidney is Correct:** In adults, approximately **85-90% of EPO** is synthesized and secreted by the **peritubular interstitial cells** (fibroblast-like cells) in the renal cortex. These cells are highly sensitive to low oxygen tension (hypoxia). When renal oxygen levels drop, Hypoxia-Inducible Factor (HIF-1α) triggers the production of EPO, which then travels to the bone marrow to stimulate the proliferation and differentiation of proerythroblasts. 2. **Why Other Options are Incorrect:** * **Pituitary:** Secretes hormones like GH, ACTH, and TSH, but has no role in EPO production. * **Lung:** While the lungs are involved in oxygenation, they do not sense hypoxia for EPO production. * **Spleen:** Acts as a reservoir for RBCs and a site for their destruction (sequestration), but does not produce EPO. **High-Yield Clinical Pearls for NEET-PG:** * **Site in Fetus:** In fetal life, the **Liver** is the primary source of EPO. Post-birth, the site shifts to the Kidneys (though the liver still produces ~10-15% in adults). * **Stimulus:** The primary stimulus for EPO secretion is **hypoxia** (not the number of RBCs). * **Clinical Correlation:** Chronic Kidney Disease (CKD) leads to a deficiency of EPO, resulting in **normocytic normochromic anemia**. Recombinant human erythropoietin (Epoetin alfa) is used to treat this. * **Polycythemia:** Renal Cell Carcinoma (RCC) can sometimes secrete excess EPO, leading to secondary polycythemia (a paraneoplastic syndrome).

Question 129: What is the half-life of Factor VIII?

A. 4 hours
B. 8 hours (Correct Answer)
C. 34 hours
D. 48 hours

Explanation: **Explanation:** **Factor VIII (Anti-Hemophilic Factor)** is a critical glycoprotein in the intrinsic pathway of the coagulation cascade. It acts as a cofactor for Factor IXa in the presence of calcium and phospholipids to activate Factor X. 1. **Why Option B is Correct:** The biological half-life of Factor VIII is approximately **8 to 12 hours**. In clinical practice and standardized examinations like NEET-PG, **8 hours** is the most commonly cited lower limit and the preferred answer. This relatively short half-life is the reason why patients with Hemophilia A require frequent infusions (often twice daily) during acute bleeding episodes or major surgery to maintain therapeutic levels. 2. **Analysis of Incorrect Options:** * **Option A (4 hours):** This is too short for Factor VIII. However, it is closer to the half-life of **Factor VII** (4–6 hours), which has the shortest half-life of all clotting factors. * **Option C (34 hours):** This is significantly longer than the half-life of Factor VIII. For comparison, **Factor IX** (deficient in Hemophilia B) has a longer half-life of approximately 18–24 hours. * **Option D (48 hours):** This is incorrect for Factor VIII. Prothrombin (Factor II) has a longer half-life of about 60–72 hours. **High-Yield Clinical Pearls for NEET-PG:** * **Shortest Half-life:** Factor VII (4–6 hours). This is why the PT/INR rises first in liver disease or Vitamin K deficiency. * **Longest Half-life:** Factor II (Prothrombin). * **Storage:** Factor VIII is unique because it is not synthesized in the liver hepatocytes but primarily in the **sinusoidal endothelial cells** of the liver and other tissues. * **Carrier Protein:** In the circulation, Factor VIII is stabilized by **von Willebrand Factor (vWF)**. A deficiency in vWF leads to a secondary decrease in Factor VIII levels because it is degraded more rapidly.

Question 130: Which of the following is NOT true regarding fetal red blood cells?

A. Elevated 2,3-DPG (Correct Answer)
B. Decreased carbonic anhydrase activity
C. Decreased life span
D. High RBC volume

Explanation: **Explanation:** The correct answer is **A (Elevated 2,3-DPG)** because fetal red blood cells (RBCs) actually have a **lower affinity for 2,3-DPG** compared to adult RBCs. **1. Why Option A is the correct (False) statement:** Fetal hemoglobin (HbF) consists of two alpha and two **gamma (γ)** chains. Unlike the beta chains in adult hemoglobin (HbA), the gamma chains lack certain positively charged amino acids, reducing their ability to bind to the negatively charged 2,3-DPG. Because 2,3-DPG normally functions to stabilize the "T-state" (deoxygenated state) and promote oxygen unloading, its inability to bind HbF results in a **higher oxygen affinity**. This shift to the left on the oxygen-dissociation curve is crucial for the fetus to extract oxygen from maternal blood across the placenta. **2. Analysis of Incorrect Options (True statements):** * **B. Decreased carbonic anhydrase activity:** Fetal RBCs have significantly lower levels of carbonic anhydrase compared to adults. This is a high-yield physiological characteristic of neonatal blood. * **C. Decreased life span:** Fetal RBCs have a shorter lifespan (approximately **60–90 days**) compared to the 120-day lifespan of adult RBCs. This contributes to physiological jaundice in neonates. * **D. High RBC volume:** Fetal RBCs are **macrocytic** (larger Mean Corpuscular Volume) and the fetus maintains a higher hematocrit/hemoglobin level to compensate for the relatively low pO2 in utero. **High-Yield Clinical Pearls for NEET-PG:** * **P50 Value:** The P50 (partial pressure of O2 at which Hb is 50% saturated) is **lower** in the fetus (~19 mmHg) than in adults (~27 mmHg), reflecting higher affinity. * **HbF Structure:** $\alpha_2\gamma_2$. * **Switchover:** HbF is gradually replaced by HbA after birth; by 6 months of age, HbA becomes the predominant type.

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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