Blood and Immunity — MCQs

Blood and Immunity Indian Medical PG Practice Questions and MCQs

Question 331: What is the normal CD4 cell count?

A. 500 cells/cu.mm
B. 1000 cells/cu.mm (Correct Answer)
C. 200 cells/cu.mm
D. 700 cells/cu.mm

Explanation: **Explanation:** The CD4+ T-lymphocyte count is a primary indicator of immune system health. In a healthy adult, the normal range for CD4 cells is typically between **500 and 1,500 cells/cu.mm**. Among the given options, **1,000 cells/cu.mm** (Option B) represents the most accurate median value for a healthy individual. These cells are "helper" T-cells that coordinate the immune response by releasing cytokines and activating other immune cells. **Analysis of Options:** * **Option A (500 cells/cu.mm):** While this is the lower limit of the normal range, it is not the standard "average" used in clinical definitions. * **Option C (200 cells/cu.mm):** This is a critical clinical threshold. A count below 200 cells/cu.mm is the diagnostic criterion for **AIDS** in HIV-positive individuals, indicating severe immunodeficiency and high risk for opportunistic infections. * **Option D (700 cells/cu.mm):** This falls within the normal range but is less representative of the standard physiological mean (1,000) taught in core physiology and pathology textbooks. **High-Yield Clinical Pearls for NEET-PG:** * **CD4:CD8 Ratio:** In healthy individuals, the normal ratio is approximately **2:1**. In HIV/AIDS, this ratio is inverted (<1:1). * **Prognostic Marker:** The CD4 count is the best predictor of immediate risk for opportunistic infections (e.g., *Pneumocystis jirovecii* occurs when CD4 < 200). * **Physiological Variation:** CD4 counts can fluctuate based on diurnal rhythm (lowest in the morning, highest in the evening), stress, and acute infections.

Question 332: What is true about ABO blood group antigens?

A. Found on RBC membrane
B. Glycoprotein in nature
C. Highly immunogenic
D. All of the above (Correct Answer)

Explanation: The ABO blood group system is the most clinically significant system in transfusion medicine. The correct answer is **D (All of the above)** because ABO antigens possess specific biochemical and immunological properties: 1. **Found on RBC membrane:** ABO antigens are integral components of the red cell surface. They are also found on the membranes of other cells, such as vascular endothelium and platelets, and can be secreted in body fluids (in "secretors"). 2. **Glycoprotein in nature:** Chemically, these antigens are complex oligosaccharides. When attached to lipids on the RBC membrane, they are **glycolipids**; however, when found in secretions (like saliva) or attached to proteins, they are **glycoproteins**. Since the question asks about the general nature of the antigens, both forms are characteristic. 3. **Highly immunogenic:** ABO antigens are the most immunogenic of all blood group antigens. This means they provoke a strong immune response. Humans naturally possess "pre-formed" IgM antibodies (isoagglutinins) against the antigens they lack, leading to immediate and potentially fatal intravascular hemolysis if an incompatible transfusion occurs. **Clinical Pearls for NEET-PG:** * **H-Antigen:** The precursor for both A and B antigens. The **Bombay Phenotype (Oh)** lacks the H-antigen and produces anti-H antibodies. * **Inheritance:** ABO genes are located on **Chromosome 9**. * **Universal Donor/Recipient:** O negative is the universal donor (no antigens); AB positive is the universal recipient (no antibodies). * **Development:** ABO antigens appear at the 6th week of fetal life but are not fully developed at birth (reaching adult levels by age 2–4).

Question 333: All of the following occur when the blood flows through the capillaries except:

A. Increase in hematocrit
B. Hb curve shift to the left (Correct Answer)
C. Decreased protein content
D. Decrease in pH

Explanation: **Explanation:** The physiological changes occurring in systemic capillaries are driven by metabolic activity in the tissues. **Why Option B is the Correct Answer:** When blood reaches the systemic capillaries, it encounters a high concentration of $CO_2$ and $H^+$ ions produced by tissue metabolism. According to the **Bohr Effect**, an increase in $PCO_2$, $H^+$ (decreased pH), and temperature causes the Oxygen-Hemoglobin dissociation curve to **shift to the RIGHT**, not the left. A rightward shift decreases Hb-O2 affinity, facilitating the unloading of oxygen to the tissues. A shift to the left occurs in the lungs, where $CO_2$ is removed. **Analysis of Incorrect Options:** * **A. Increase in hematocrit:** As blood flows through capillaries, the "Chloride Shift" (Hamburger phenomenon) occurs. $HCO_3^-$ leaves the RBCs while $Cl^-$ enters. This increases the osmotic pressure inside the RBC, causing water to enter and the cell to swell. Consequently, the Mean Corpuscular Volume (MCV) increases, leading to a slightly higher hematocrit in venous blood compared to arterial blood. * **C. Decreased protein content:** Due to hydrostatic pressure, a small amount of protein-free fluid (ultrafiltrate) filters out into the interstitial space. While most is reabsorbed, the net effect is a slight decrease in the protein concentration of the blood leaving the capillary bed. * **D. Decrease in pH:** Tissues produce $CO_2$, which reacts with water to form carbonic acid ($H_2CO_3$), dissociating into $H^+$ and $HCO_3^-$. This increase in hydrogen ion concentration results in a lower (more acidic) pH in capillary and venous blood. **High-Yield Pearls for NEET-PG:** * **CADET, face Right:** Factors shifting the curve to the **Right** include **C**O2, **A**cidosis, **D**PG (2,3-BPG), **E**xercise, and **T**emperature. * **Chloride Shift:** $Cl^-$ moves **into** RBCs in systemic capillaries and **out** of RBCs in pulmonary capillaries. * Venous blood hematocrit is typically **3% higher** than arterial blood hematocrit due to RBC swelling.

Question 334: At what age is fetal hemoglobin completely replaced by adult hemoglobin?

A. At birth
B. 2 months
C. 4 months
D. 6 months (Correct Answer)

Explanation: **Explanation:** The transition from fetal hemoglobin (HbF, $\alpha_2\gamma_2$) to adult hemoglobin (HbA, $\alpha_2\beta_2$) is a physiological process known as **hemoglobin switching**. HbF has a higher affinity for oxygen, which is essential for extracting oxygen from maternal blood in the placenta. 1. **Why 6 months is correct:** The synthesis of $\gamma$-chains (gamma) begins to decline during the third trimester of pregnancy, while $\beta$-chain (beta) synthesis increases. At birth, HbF constitutes about 60-80% of total hemoglobin. This level drops significantly over the first few months of life. By **6 months of age**, HbF is almost entirely replaced by HbA, reaching the stable adult level of <1%. 2. **Why other options are incorrect:** * **At birth:** HbF is still the predominant hemoglobin (approx. 75%). * **2 to 4 months:** This is the period of "physiological anemia of infancy" where HbF is rapidly declining and HbA is rising, but the replacement is not yet complete. **Clinical Pearls for NEET-PG:** * **Structure:** HbF = $\alpha_2\gamma_2$; HbA = $\alpha_2\beta_2$; HbA2 = $\alpha_2\delta_2$. * **2,3-BPG:** HbF has a poor binding affinity for 2,3-BPG, which is why its oxygen dissociation curve is shifted to the **left** compared to HbA. * **Clinical Significance:** Conditions like **Sickle Cell Anemia** and **$\beta$-Thalassemia** often become symptomatic only after 6 months of age, coinciding with the disappearance of protective HbF. * **Induction:** Hydroxyurea is used in Sickle Cell Disease because it increases the production of HbF, which inhibits the polymerization of HbS.

Question 335: Which of the following clotting factors are vitamin K dependent?

A. Factor IX and X (Correct Answer)
B. Factor IV
C. Factor XII
D. Factor I

Explanation: **Explanation:** Vitamin K is essential for the post-translational modification of specific clotting factors. It acts as a cofactor for the enzyme **gamma-glutamyl carboxylase**, which adds a carboxyl group to glutamate residues on these proteins. This modification allows the factors to bind calcium ions ($Ca^{2+}$) and attach to phospholipid surfaces, a crucial step in the coagulation cascade. **Correct Option (A): Factor IX and X** The Vitamin K-dependent clotting factors are **Factors II (Prothrombin), VII, IX, and X**. Additionally, the anticoagulant proteins **Protein C and Protein S** are also Vitamin K-dependent. Therefore, Factor IX (Christmas factor) and Factor X (Stuart-Prower factor) are the correct choices. **Incorrect Options:** * **Factor IV (Option B):** This is simply **Calcium ions**. While calcium is required for the activation of Vitamin K-dependent factors, it is not synthesized or modified by Vitamin K. * **Factor XII (Option C):** Known as the **Hageman factor**, it is part of the intrinsic pathway (contact activation) and is not dependent on Vitamin K for its synthesis. * **Factor I (Option D):** This is **Fibrinogen**. It is synthesized in the liver but does not require Vitamin K-mediated carboxylation. **High-Yield Clinical Pearls for NEET-PG:** * **Warfarin (Oral Anticoagulant):** Acts by inhibiting **Vitamin K Epoxide Reductase (VKOR)**, preventing the recycling of Vitamin K and thus inhibiting the synthesis of Factors II, VII, IX, and X. * **Shortest Half-life:** Factor VII has the shortest half-life among these factors, which is why the **Prothrombin Time (PT)** is the first to be prolonged in Vitamin K deficiency or Warfarin therapy. * **Newborns:** They are Vitamin K deficient due to a sterile gut and poor placental transfer; hence, a prophylactic Vitamin K injection is given at birth to prevent **Hemorrhagic Disease of the Newborn**.

Question 336: Which of the following coagulation factors causes cross-linking and stabilization of a clot?

A. Factor XIII (Correct Answer)
B. Thrombin
C. Factor VIII
D. Factor IX

Explanation: ### Explanation **Correct Answer: A. Factor XIII** **Mechanism:** Factor XIII, also known as **Fibrin Stabilizing Factor** (or Laki-Lorand factor), is the final enzyme in the coagulation cascade. While thrombin converts soluble fibrinogen into fibrin monomers, these monomers are initially held together by weak hydrogen bonds (forming a "soft clot"). Factor XIII is activated by thrombin in the presence of calcium to become **Factor XIIIa**. It then acts as a transglutaminase, creating covalent cross-links between the fibrin strands. This process transforms the fragile network into a stable, insoluble "hard clot" that is resistant to premature lysis. **Why the other options are incorrect:** * **B. Thrombin (Factor IIa):** Its primary role is converting fibrinogen to fibrin and activating Factors V, VIII, XI, and XIII. It initiates clot formation but does not perform the cross-linking itself. * **C. Factor VIII (Anti-hemophilic Factor A):** This is a cofactor for Factor IXa in the intrinsic pathway. Its deficiency causes Hemophilia A. * **D. Factor IX (Christmas Factor):** This is a serine protease in the intrinsic pathway. Its deficiency causes Hemophilia B. **High-Yield Clinical Pearls for NEET-PG:** * **Screening Tests:** Standard coagulation tests like PT and aPTT are **normal** in Factor XIII deficiency because they measure the time to initial fibrin formation, not the strength of the cross-linking. * **Diagnosis:** Factor XIII deficiency is diagnosed using the **Urea Solubility Test** (a clot that dissolves in 5M urea or 1% monochloroacetic acid indicates deficiency). * **Clinical Presentation:** Characterized by delayed bleeding (e.g., umbilical cord stump bleeding in neonates) and poor wound healing.

Question 337: In which of the following conditions does the oxygen dissociation curve shift to the right?

A. Hyperkalemia
B. Hypokalemia
C. Metabolic alkalosis
D. Anemia (Correct Answer)

Explanation: **Explanation:** The oxygen dissociation curve (ODC) represents the relationship between the partial pressure of oxygen ($PO_2$) and the percentage saturation of hemoglobin. A **shift to the right** indicates a decreased affinity of hemoglobin for oxygen, facilitating oxygen unloading to the tissues. **Why Anemia is correct:** In chronic anemia, there is a compensatory increase in the production of **2,3-Bisphosphoglycerate (2,3-BPG)** within red blood cells. 2,3-BPG binds to the beta chains of deoxyhemoglobin, stabilizing the "T" (tense) state and decreasing oxygen affinity. This shifts the ODC to the right, allowing the limited amount of hemoglobin to deliver oxygen more efficiently to hypoxic tissues. **Analysis of Incorrect Options:** * **Hyperkalemia/Hypokalemia:** Potassium levels do not directly affect the hemoglobin-oxygen affinity. While severe acid-base imbalances (which shift the curve) can cause potassium shifts, potassium itself is not a primary determinant of the ODC. * **Metabolic Alkalosis:** An increase in pH (alkalosis) causes a **left shift** (Bohr Effect). A decrease in hydrogen ion concentration increases hemoglobin’s affinity for oxygen, making it harder for tissues to transition oxygen from the blood. **High-Yield Clinical Pearls for NEET-PG:** To remember the causes of a **Right Shift**, use the mnemonic **"CADET, face Right!"**: * **C:** **C**O2 increase (Hypercapnia) * **A:** **A**cidosis (Increased $H^+$ / Decreased pH) * **D:** **D**PG (2,3-BPG increase) * **E:** **E**xercise * **T:** **T**emperature increase *Note: Fetal hemoglobin (HbF) causes a **Left Shift** because it has a lower affinity for 2,3-BPG compared to adult hemoglobin (HbA).*

Question 338: What is embryonic hemoglobin?

A. Adult hemoglobin
B. Fetal hemoglobin
C. Gower hemoglobin (Correct Answer)
D. None of the above

Explanation: ### Explanation **Correct Answer: C. Gower hemoglobin** **Why it is correct:** Embryonic hemoglobin refers to the hemoglobin variants synthesized during the first trimester of intrauterine life (specifically from the 3rd to the 10th week of gestation). These are produced in the **yolk sac** before the liver and bone marrow take over erythropoiesis. There are three primary types of embryonic hemoglobin: 1. **Gower 1:** ($\zeta_2\epsilon_2$) 2. **Gower 2:** ($\alpha_2\epsilon_2$) 3. **Portland:** ($\zeta_2\gamma_2$) Since Gower hemoglobin is the classic representative of this stage, it is the correct choice. **Why other options are incorrect:** * **A. Adult hemoglobin (HbA):** This consists of two alpha ($\alpha$) and two beta ($\beta$) chains ($\alpha_2\beta_2$). It begins to appear at around 30 weeks of gestation but only becomes the predominant form after birth (around 6 months of age). * **B. Fetal hemoglobin (HbF):** This consists of two alpha ($\alpha$) and two gamma ($\gamma$) chains ($\alpha_2\gamma_2$). It is the primary hemoglobin from the late first trimester until birth. It has a higher affinity for oxygen than HbA to facilitate oxygen transfer across the placenta. **High-Yield Clinical Pearls for NEET-PG:** * **Site of Erythropoiesis:** Yolk sac (3–10 weeks) $\rightarrow$ Liver (6 weeks–birth; peak at 4 months) $\rightarrow$ Spleen (10–28 weeks) $\rightarrow$ Bone Marrow (from 18 weeks onwards). * **Chain Transition:** The $\zeta$ (zeta) and $\epsilon$ (epsilon) chains are replaced by $\alpha$ (alpha) and $\gamma$ (gamma) chains as the fetus develops. * **P50 Value:** HbF has a lower P50 (approx. 19 mmHg) compared to HbA (approx. 27 mmHg), reflecting its higher oxygen affinity. * **HbA2:** A minor adult hemoglobin ($\alpha_2\delta_2$) normally comprising <3% of total hemoglobin.

Question 339: Which of the following surface proteins is most often expressed in human hematopoietic stem cells?

A. CD22
B. CD45
C. CD15
D. CD34 (Correct Answer)

Explanation: **Explanation:** **Hematopoietic Stem Cells (HSCs)** are multipotent cells found primarily in the bone marrow that give rise to all blood cell lineages. The most characteristic and clinically significant surface marker for identifying and isolating these cells is **CD34**. * **CD34 (Correct Answer):** This is a cell surface glycoprotein that acts as an adhesion molecule. It is highly expressed on HSCs and early hematopoietic progenitor cells. As these cells mature and differentiate, CD34 expression is lost. In clinical practice, CD34+ cell counts are used to quantify stem cells for **bone marrow transplantation**. **Analysis of Incorrect Options:** * **CD22:** This is a specific marker for **B-lineage cells**. It is found on mature B-cells and is used in the diagnosis of B-cell leukemias and lymphomas. * **CD45:** Known as the **Leukocyte Common Antigen (LCA)**, it is expressed on all white blood cells. While HSCs do express CD45, it is not specific to stem cells as it persists throughout all stages of leukocyte maturation. * **CD15:** This is a marker primarily associated with **Reed-Sternberg cells** (in Hodgkin Lymphoma) and mature granulocytes (neutrophils). **High-Yield Clinical Pearls for NEET-PG:** * **Stem Cell Isolation:** CD34 is the "gold standard" marker used for **flow cytometry** to ensure an adequate graft dose during peripheral blood stem cell harvesting. * **Other HSC Markers:** HSCs are typically described as **CD34+, CD38–, and Lin–** (lineage negative). * **CD117 (c-kit):** Another important marker found on hematopoietic stem cells and mast cells. * **CD30 & CD15:** Classic markers for **Hodgkin Lymphoma** (Reed-Sternberg cells).

Question 340: In the red blood cell series, at which stage does proliferation occur?

A. Early normoblast (Correct Answer)
B. Intermediate normoblast
C. Late normoblast
D. Proerythroblast

Explanation: ### Explanation The process of erythropoiesis involves both **proliferation** (cell division) and **differentiation** (maturation). The ability of a cell to undergo mitosis decreases as it matures and accumulates hemoglobin. **Why Early Normoblast is the Correct Answer:** In the red blood cell series, the **Early Normoblast** (also known as the Basophilic Erythroblast) is the stage characterized by the **highest rate of proliferation**. While the proerythroblast also divides, the early normoblast stage involves active mitotic activity to significantly expand the cell population before the nucleus begins to condense and eventually degenerate. **Analysis of Incorrect Options:** * **D. Proerythroblast:** This is the first identifiable cell of the RBC series. While it does undergo mitosis, the bulk of numerical expansion (proliferation) occurs in the subsequent normoblast stages. * **B. Intermediate Normoblast (Polychromatic):** At this stage, hemoglobin starts appearing. While some mitosis still occurs here, it is the **last stage** capable of cell division. * **C. Late Normoblast (Orthochromatic):** At this stage, the nucleus becomes pyknotic (shrunken and condensed). **No proliferation occurs** here; the cell is purely maturing and preparing to extrude its nucleus to become a reticulocyte. **High-Yield Clinical Pearls for NEET-PG:** * **Last stage of cell division:** Intermediate Normoblast. * **Last nucleated stage:** Late Normoblast (the nucleus is extruded at the end of this stage). * **Hemoglobin appearance:** First starts appearing in the **Intermediate Normoblast**, though it is functionally present in the Late Normoblast. * **Reticulocyte:** Contains remnants of ribosomal RNA (Golgi apparatus), which gives it a "reticular" appearance under supravital stains like New Methylene Blue. * **Erythropoietin (EPO):** Primarily acts on the CFU-E (Colony Forming Unit-Erythroid) and Proerythroblasts to stimulate production.

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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