Blood and Immunity — MCQs

Blood and Immunity Indian Medical PG Practice Questions and MCQs

Question 51: Which of the following best assesses platelet function?

A. Platelet adhesion (Correct Answer)
B. Bleeding time
C. Clotting time
D. Prothrombin time

Explanation: **Explanation:** The assessment of platelet function focuses on the ability of platelets to perform their primary roles: **adhesion, aggregation, and secretion**. **1. Why Platelet Adhesion is the Correct Answer:** Platelet adhesion is the critical first step in primary hemostasis, where platelets attach to the exposed subendothelial collagen via **von Willebrand Factor (vWF)** and the **GpIb-IX-V receptor**. Tests that measure adhesion (like the Glass Bead Column test or Platelet Function Analyzers) directly evaluate the intrinsic functional capacity of the platelets to initiate a plug. While "Platelet Aggregation" is also a functional test, among the given options, **adhesion** represents the fundamental qualitative function of the cell. **2. Analysis of Incorrect Options:** * **Bleeding Time (BT):** While BT is a screening test for primary hemostasis, it is **non-specific**. It is affected by platelet count (quantitative), platelet function (qualitative), and vascular integrity. It is no longer the "gold standard" due to poor reproducibility. * **Clotting Time (CT):** This measures the **intrinsic and common pathways** of the coagulation cascade (secondary hemostasis). It does not assess platelet function. * **Prothrombin Time (PT):** This assesses the **extrinsic and common pathways** (Factors VII, X, V, II, and I). It is used to monitor Warfarin therapy and liver function, not platelets. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard for Platelet Function:** Light Transmission Aggregometry (LTA). * **Bernard-Soulier Syndrome:** Defect in platelet **adhesion** (GpIb receptor deficiency). * **Glanzmann Thrombasthenia:** Defect in platelet **aggregation** (GpIIb/IIIa receptor deficiency). * **vWF Disease:** The most common inherited bleeding disorder; it impairs **adhesion** because vWF acts as the "glue" between the vessel wall and the platelet.

Question 52: Polycythemia is typically associated with which of the following haematocrit values?

A. 60% in Men; > 55% in Women (Correct Answer)
B. 60% in Women; > 55% in Men
C. 55% in Men; > 50% in Women
D. 50% in Women; > 55% in Men

Explanation: **Explanation:** **Polycythemia** is defined as an increase in the total red blood cell mass, which clinically manifests as an elevation in hemoglobin (Hb) and hematocrit (Hct) levels. **1. Why Option A is Correct:** The physiological reference range for hematocrit is higher in men than in women due to the stimulatory effect of androgens on erythropoietin production. According to standard physiological textbooks (like Guyton and Hall), the normal Hct is approximately **47% (±5) for men** and **42% (±5) for women**. Polycythemia is diagnosed when these values significantly exceed the upper limit of normal. Specifically, a hematocrit **>60% in men** and **>55% in women** is the classic threshold used to define absolute polycythemia. **2. Why Other Options are Incorrect:** * **Option B:** Reverses the gender values. Men naturally have higher Hct levels; therefore, the threshold for polycythemia must be higher for men than for women. * **Options C & D:** These values (50–55%) represent the "borderline" or "high-normal" range. While they may indicate mild erythrocytosis, they do not meet the definitive diagnostic criteria for polycythemia used in standardized medical examinations. **Clinical Pearls for NEET-PG:** * **Primary Polycythemia (Polycythemia Vera):** A myeloproliferative neoplasm associated with the **JAK2 V617F mutation**. It features low Erythropoietin (EPO) levels. * **Secondary Polycythemia:** Driven by high EPO levels, often due to chronic hypoxia (high altitude, COPD, cyanotic heart disease) or EPO-secreting tumors (Renal Cell Carcinoma). * **Relative Polycythemia (Gaisbock syndrome):** Elevated Hct due to decreased plasma volume (dehydration) rather than increased RBC mass. * **Hyperviscosity:** When Hct exceeds 60-70%, blood viscosity increases sharply, leading to risks of thrombosis and stroke.

Question 53: Bleeding time is prolonged in which of the following conditions?

A. Haemophilia
B. Christmas disease
C. Von-Willebrand disease (Correct Answer)
D. Vitamin K-deficiency

Explanation: **Explanation:** The **Bleeding Time (BT)** is a clinical test that assesses the **primary hemostatic pathway**, which involves the interaction between the vascular wall and platelets (platelet plug formation). **1. Why Von-Willebrand Disease (vWD) is correct:** Von Willebrand Factor (vWF) is essential for platelet adhesion to the subendothelial collagen via the GpIb receptor. In vWD, the deficiency or dysfunction of vWF leads to defective platelet adhesion. Since BT measures the time taken for a platelet plug to form, it is characteristically **prolonged** in vWD. Additionally, vWF acts as a carrier protein for Factor VIII; thus, vWD is unique because it can show both a prolonged BT and a prolonged aPTT. **2. Why the other options are incorrect:** * **Haemophilia A (Factor VIII deficiency) & Christmas Disease (Haemophilia B/Factor IX deficiency):** These are disorders of the **secondary hemostatic pathway** (coagulation cascade). In these conditions, platelet function is normal, so the Bleeding Time is **normal**. However, the Clotting Time (CT) and aPTT are prolonged. * **Vitamin K Deficiency:** Vitamin K is required for the gamma-carboxylation of Factors II, VII, IX, and X. This affects the coagulation cascade, leading to a prolonged Prothrombin Time (PT) and aPTT, but the **Bleeding Time remains normal** as platelet function is unaffected. **NEET-PG High-Yield Pearls:** * **BT** = Platelet function/number (Prolonged in Thrombocytopenia, vWD, Bernard-Soulier). * **CT/aPTT** = Coagulation factors (Prolonged in Haemophilia, Vitamin K deficiency). * **vWD** is the most common inherited bleeding disorder. * **Bernard-Soulier Syndrome** also shows prolonged BT and is often confused with vWD, but it involves a deficiency of the GpIb receptor itself.

Question 54: Which cell type is primarily concerned with cell-mediated immunity?

A. B-Lymphocytes
B. T-Lymphocytes (Correct Answer)
C. Eosinophils
D. Monocytes

Explanation: **Explanation:** The correct answer is **T-Lymphocytes**. Immunity is broadly divided into two arms: Humoral (Antibody-mediated) and Cell-Mediated Immunity (CMI). **1. Why T-Lymphocytes are correct:** T-lymphocytes (T-cells) are the primary mediators of **Cell-Mediated Immunity**. Unlike B-cells, they do not produce antibodies. Instead, they recognize antigens presented by Major Histocompatibility Complex (MHC) molecules on the surface of cells. * **CD8+ T-cells (Cytotoxic):** Directly destroy virally infected or cancerous cells. * **CD4+ T-cells (Helper):** Orchestrate the immune response by secreting cytokines that activate other immune cells. **2. Why other options are incorrect:** * **B-Lymphocytes:** These are responsible for **Humoral Immunity**. Upon activation, they differentiate into plasma cells that secrete antibodies (Immunoglobulins) into the blood and lymph. * **Eosinophils:** These are granulocytes primarily involved in combating **parasitic infections** and mediating **allergic reactions** (Type I Hypersensitivity). * **Monocytes:** These are phagocytic cells that circulate in the blood. Once they migrate into tissues, they become **macrophages**, acting as professional Antigen-Presenting Cells (APCs) rather than the primary effectors of CMI. **Clinical Pearls for NEET-PG:** * **Origin & Maturation:** All lymphocytes originate in the bone marrow, but T-cells mature in the **Thymus** (hence "T"-cells). * **MHC Restriction:** CD4+ cells recognize MHC Class II, while CD8+ cells recognize MHC Class I (Rule of 8: 4×2=8 and 8×1=8). * **Clinical Correlation:** In HIV/AIDS, there is a progressive depletion of **CD4+ T-cells**, leading to a collapse of cell-mediated immunity and increased susceptibility to opportunistic infections.

Question 55: Which of the following is NOT a platelet aggregator?

A. Adenosine diphosphate (ADP)
B. Thromboxane
C. Steroids
D. Prostaglandin D2 (PGD2) (Correct Answer)

Explanation: **Explanation:** Platelet aggregation is a tightly regulated process involving a balance between pro-aggregatory (pro-thrombotic) and anti-aggregatory (anti-thrombotic) substances. **Why Prostaglandin D2 (PGD2) is the correct answer:** PGD2 is a potent **inhibitor** of platelet aggregation. It acts by binding to the DP1 receptor on the platelet surface, which stimulates adenylate cyclase to increase intracellular cyclic AMP (cAMP) levels. Elevated cAMP prevents platelet activation and aggregation. Other potent inhibitors include **Prostacyclin (PGI2)** and **Nitric Oxide (NO)**. **Analysis of Incorrect Options:** * **ADP (Adenosine diphosphate):** This is a powerful physiological aggregator. It is released from platelet dense granules and binds to P2Y1 and P2Y12 receptors, leading to further platelet recruitment. * **Thromboxane (TXA2):** Produced via the COX-1 pathway, TXA2 is one of the most potent platelet aggregators and vasoconstrictors. It is the primary target of Aspirin (which irreversibly inhibits COX-1). * **Steroids:** While not primary physiological aggregators like ADP, steroids (specifically glucocorticoids) can increase platelet count and enhance platelet reactivity/aggregation in certain clinical contexts, making them "pro-aggregatory" rather than inhibitors. **NEET-PG High-Yield Pearls:** * **The "Rule of cAMP":** In platelets, **High cAMP = Inhibition** of aggregation. In contrast, **Low cAMP/High Calcium = Activation**. * **PGI2 vs. TXA2:** These are functional antagonists. PGI2 (from endothelium) inhibits aggregation; TXA2 (from platelets) promotes it. * **Platelet Granules:** ADP is stored in **Dense granules** (along with Calcium and Serotonin), while vWF and Fibrinogen are in **Alpha granules**.

Question 56: Blood group antigens are chemically:

A. Carbohydrate
B. Glycoprotein (Correct Answer)
C. Phospholipids
D. Polysaccharide

Explanation: ### Explanation **Correct Answer: B. Glycoprotein** **Why Glycoprotein is Correct:** Blood group antigens (such as ABO and Rh) are biochemical markers located on the surface of the erythrocyte membrane. While the **antigenic specificity** (the part that determines the blood type) is often determined by specific sugar sequences (carbohydrates), the **entire chemical structure** of the antigen molecule anchored to the red cell membrane is a **glycoprotein** or a **glycolipid**. In the context of standard medical examinations like NEET-PG, these antigens are primarily classified as glycoproteins because they consist of a polypeptide backbone with attached oligosaccharide chains. **Analysis of Incorrect Options:** * **A. Carbohydrate:** While the terminal sugars (e.g., N-acetylgalactosamine for Group A, Galactose for Group B) define the immunological specificity, the carbohydrate does not exist in isolation; it is attached to a protein or lipid base. * **C. Phospholipids:** These form the structural bilayer of the cell membrane but do not function as the primary ABO or Rh antigens. * **D. Polysaccharide:** This term refers to long chains of monosaccharides. While the antigenic portion is an oligosaccharide, the molecule as a whole is a conjugated protein. **High-Yield Clinical Pearls for NEET-PG:** * **ABO Antigens:** These are found not only on RBCs but also in most body fluids (if the person is a "secretor"). They are primarily **Glycoproteins** in secretions and **Glycolipids/Glycoproteins** on RBC membranes. * **Rh Antigen:** Unlike ABO, the Rh antigen is a **pure protein** (non-glycosylated) and is found exclusively on the RBC membrane. * **H-Antigen:** This is the precursor molecule. The *H gene* codes for **Fucosyltransferase**, which adds L-fucose to the precursor chain. * **Bombay Blood Group:** Characterized by the absence of the H-antigen (genotype *hh*). These individuals produce anti-H antibodies.

Question 57: Which compound shifts the oxygen dissociation curve to the right?

A. 1-Phosphoglycerate
B. 2,3 DPG (Correct Answer)
C. 1,3 DPG
D. Glyceraldehyde

Explanation: **Explanation:** The **Oxygen Dissociation Curve (ODC)** represents the relationship between the partial pressure of oxygen ($PO_2$) and the percentage saturation of hemoglobin ($Hb$). A **shift to the right** indicates a decreased affinity of hemoglobin for oxygen, meaning oxygen is more easily unloaded to the tissues. **Why 2,3-DPG is Correct:** 2,3-Diphosphoglycerate (2,3-DPG), also known as 2,3-BPG, is a metabolic byproduct of glycolysis in erythrocytes (via the Rapoport-Luebering shunt). It binds to the beta chains of deoxyhemoglobin, stabilizing the **T-state (Tense state)**. This reduces hemoglobin's affinity for oxygen, shifting the curve to the right and facilitating oxygen delivery to peripheral tissues. **Analysis of Incorrect Options:** * **A & C (1-Phosphoglycerate and 1,3-DPG):** These are intermediate metabolites in the Embden-Meyerhof glycolytic pathway. While 1,3-DPG is the precursor to 2,3-DPG, it does not directly bind to hemoglobin to modulate oxygen affinity. * **D (Glyceraldehyde):** This is a simple monosaccharide (triose) involved in carbohydrate metabolism but has no direct physiological role in shifting the ODC. **NEET-PG High-Yield Pearls:** * **Mnemonic for Right Shift (CADET, face Right!):** **C**O2 increase, **A**cidosis ($H^+$), **D**PG (2,3-DPG) increase, **E**xercise, **T**emperature increase. * **Fetal Hemoglobin (HbF):** Shifts the curve to the **Left** because it has a lower affinity for 2,3-DPG (due to gamma chains instead of beta chains), allowing the fetus to pull oxygen from maternal blood. * **Stored Blood:** Levels of 2,3-DPG decrease in stored blood, causing a **Left shift**, which impairs oxygen delivery post-transfusion.

Question 58: Antithrombin is a protein produced by which organ?

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

Explanation: **Explanation:** **Antithrombin (specifically Antithrombin III)** is a small glycoprotein and a potent natural anticoagulant. It is primarily synthesized in the **Liver**. Its main function is to inhibit the coagulation cascade by inactivating several serine proteases, most notably **Thrombin (Factor IIa)** and **Factor Xa**, as well as factors IXa, XIa, and XIIa. * **Why Liver is Correct:** The liver is the "metabolic factory" responsible for synthesizing almost all coagulation factors and their inhibitors (except Factor VIII and von Willebrand factor). Antithrombin is constitutively produced by hepatocytes and released into the plasma. * **Why other options are incorrect:** * **Spleen:** Primarily involved in the filtration of aged red blood cells and immune surveillance; it does not synthesize plasma proteins like antithrombin. * **Kidney:** While the kidney produces Erythropoietin and Renin, it does not produce antithrombin. However, in nephrotic syndrome, antithrombin can be lost in the urine, leading to a hypercoagulable state. * **Lung:** The lungs are involved in converting Angiotensin I to II and producing surfactant, but they are not a source of antithrombin. **High-Yield Clinical Pearls for NEET-PG:** 1. **Mechanism of Action:** Antithrombin’s activity is increased **1,000 to 2,000 times** in the presence of **Heparin**. This is the physiological basis for heparin therapy. 2. **Heparin Resistance:** Patients with a congenital or acquired deficiency of Antithrombin III will show "heparin resistance," meaning standard doses of heparin fail to prolong the PTT significantly. 3. **Clinical Correlation:** Deficiency of Antithrombin (inherited or acquired via liver disease/nephrotic syndrome) leads to a high risk of venous thromboembolism (VTE).

Question 59: Which complement components are produced by the spleen?

A. C1
B. C2, C4
C. C5, C8 (Correct Answer)
D. C3, C6

Explanation: ### Explanation The complement system consists of proteins primarily synthesized in the **liver** (over 90%), but extrahepatic synthesis occurs in various tissues, including the spleen, macrophages, and epithelial cells. **Why C5 and C8 are the correct answer:** While the liver is the "factory" for most complement proteins, the **spleen** is a significant site for the synthesis of specific components, most notably **C5, C6, C8, and C9**. These are late-acting components involved in the formation of the Membrane Attack Complex (MAC). Studies in asplenic individuals and organ-specific assays have confirmed that the splenic parenchyma contributes significantly to the circulating levels of C5 and C8. **Analysis of Incorrect Options:** * **A. C1:** C1 (specifically C1q, C1r, and C1s) is primarily produced by **intestinal epithelial cells** and macrophages. * **B. C2, C4:** These are early components of the classical pathway. They are synthesized predominantly in the **liver**, though macrophages also produce them in smaller quantities. * **D. C3, C6:** While the spleen does produce some C6, **C3** is the most abundant complement protein and is almost exclusively synthesized by **hepatocytes**. **High-Yield NEET-PG Pearls:** * **Primary Site:** The liver is the primary source of C3, C4, C5, C6, C9, and Factor B. * **Smallest Complement:** C8 is unique because it is a complex of three different polypeptide chains (alpha, beta, gamma). * **Clinical Correlation:** Patients with **splenectomy** or hyposplenism have a slightly altered complement profile, contributing to their increased susceptibility to encapsulated organisms (e.g., *S. pneumoniae*), though the loss of splenic macrophages and opsonization (Tuftsin/Properdin) is the more dominant factor. * **C1 Synthesis:** Remember "C1 = Colon/Intestine" for quick recall.

Question 60: Conversion of prothrombin to thrombin requires:

A. Factor V only
B. Factor V and Ca++
C. Factor XII
D. Factor X and Ca++ (Correct Answer)

Explanation: ### Explanation The conversion of prothrombin (Factor II) to thrombin (Factor IIa) is the pivotal step in the **Common Pathway** of the coagulation cascade. **1. Why Option D is Correct:** The conversion is catalyzed by the **Prothrombinase Complex**. This complex consists of: * **Factor Xa** (The active enzyme) * **Factor Va** (The cofactor) * **Calcium ions (Ca++)** * **Phospholipids** (from platelets) While Factor V is necessary, **Factor Xa** is the primary protease that cleaves prothrombin. Calcium is essential because it acts as a bridge, anchoring the clotting factors to the phospholipid surface of activated platelets via gamma-carboxyglutamic acid residues. **2. Why Other Options are Incorrect:** * **Option A & B:** Factor V alone or with Calcium cannot convert prothrombin. Factor V acts only as a **cofactor** that accelerates the activity of Factor Xa by several thousand-fold. Without the protease (Factor X), the reaction does not occur. * **Option C:** Factor XII (Hageman factor) is involved in the initiation of the **Intrinsic Pathway**. It activates Factor XI but has no direct role in the final conversion of prothrombin to thrombin. **3. NEET-PG High-Yield Pearls:** * **Vitamin K Dependency:** Factors II, VII, IX, and X require Vitamin K for gamma-carboxylation, which allows them to bind **Calcium**. * **Rate-Limiting Step:** The formation of the Prothrombinase complex is often considered the "bottleneck" or rate-limiting step of coagulation. * **Thrombin’s Roles:** Once formed, thrombin not only converts fibrinogen to fibrin but also provides **positive feedback** by activating Factors V, VIII, and XI. * **Lab Correlation:** The Prothrombin Time (PT) measures the Extrinsic and Common pathways, while aPTT measures the Intrinsic and Common pathways. Both will be prolonged if Factor X or Prothrombin is deficient.

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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