Blood and Immunity — MCQs

Blood and Immunity Indian Medical PG Practice Questions and MCQs

Question 471: Cell type which lacks HLA antigen is:

A. Monocytes
B. Platelets
C. Neutrophils
D. Erythrocytes (Correct Answer)

Explanation: ***Erythrocytes*** - Mature **erythrocytes** (red blood cells) lack a nucleus and therefore do not express **MHC/HLA antigens** on their surface. This is critical for their role in oxygen transport without triggering immune responses. - While other blood cells like leukocytes express HLA antigens, erythrocytes are a notable exception, making them less immunogenic in certain contexts. *Monocytes* - **Monocytes** are **antigen-presenting cells** and express both **MHC Class I and Class II** molecules (HLA-A, B, C, DR, DP, DQ) on their surface. - These HLA antigens are crucial for monocytes to present antigens to T lymphocytes and initiate immune responses. *Platelets* - **Platelets** express **MHC Class I (HLA-A, B, C)** antigens on their surface. - These HLA antigens can be a target for alloantibodies, contributing to conditions like **post-transfusion purpura** and **neonatal alloimmune thrombocytopenia**. *Neutrophils* - **Neutrophils** are granulocytes that express **MHC Class I (HLA-A, B, C)** antigens on their surface. - While their primary role is phagocytosis, the presence of HLA Class I allows them to be recognized by the immune system and can be relevant in transfusion reactions.

Question 472: The primary vasoactive mediator present in mast cells is

A. Histamine (Correct Answer)
B. Tryptase
C. Heparin
D. Chymase

Explanation: ***Histamine*** - **Histamine** is a primary **vasoactive amine** stored in the granules of **mast cells** and **basophils**. - Upon mast cell activation, histamine is rapidly released, causing **vasodilatation**, increased **vascular permeability**, and **bronchoconstriction**; it is a key mediator in immediate hypersensitivity reactions. *Heparin* - **Heparin** is an **anticoagulant proteoglycan** found in mast cell granules, but it is not considered the primary vasoactive mediator. - Its main function is to prevent **blood clotting**, which is distinct from the immediate allergic responses mediated by histamine. *Tryptase* - **Tryptase** is a **neutral protease** found almost exclusively in human **mast cells** and is released upon their activation. - While it is a valuable marker for mast cell activation and plays a role in tissue remodeling, it is an **enzyme** rather than a primary vasoactive mediator like histamine. *Chymase* - **Chymase** is another **neutral protease** stored in **mast cell granules**, similar to tryptase. - It participates in **extracellular matrix degradation** and **angiotensin conversion** but is not the main mediator responsible for the immediate vascular and smooth muscle effects seen in allergic reactions.

Question 473: Lymphoid tissues reach their maximum size:

A. At 20 years of age
B. In early childhood
C. During adolescence
D. At puberty (Correct Answer)

Explanation: ***At puberty*** - Lymphoid tissues, including the **thymus**, **lymph nodes**, **spleen**, and **tonsils**, exhibit a growth pattern characterized by rapid expansion during childhood and reaching their peak size around the time of **puberty (approximately 12-13 years of age)**. - This growth reflects the active development and maturation of the **immune system** during these formative years. - According to **Scammon's growth curves**, lymphoid tissue follows a unique pattern, growing to nearly **200% of adult size** before involuting. *In early childhood* - While lymphoid tissues are actively growing during early childhood, their **maximum size** is typically attained later, closer to puberty. - Early childhood is a period of significant immune system development, but not the peak of lymphoid tissue mass. *During adolescence* - After reaching their maximum size around puberty, lymphoid tissues, particularly the **thymus**, begin to undergo **involution** during adolescence. - This means they start to decrease in size and mass rather than continuing to grow. *At 20 years of age* - By the age of 20, lymphoid tissues, especially the **thymus**, have significantly **regressed** in size due to age-related involution. - The immune system is mature, but the total mass of lymphoid tissue is considerably less than its peak at puberty.

Question 474: Endothelial cells synthesize:

A. Von Willebrand Factor (Correct Answer)
B. Factor XII
C. Factor X
D. Fibrinogen

Explanation: ***Von Willebrand Factor*** - **Endothelial cells** are the primary site of synthesis and storage for **Von Willebrand factor (vWF)**. - **vWF** is crucial for primary hemostasis, mediating **platelet adhesion** to subendothelial collagen and acting as a carrier protein for **Factor VIII**. *Factor X* - **Factor X** (Stuart-Prower factor) is a **vitamin K-dependent coagulation factor** synthesized primarily in the **liver**. - It plays a central role in both the intrinsic and extrinsic pathways, ultimately converting **prothrombin to thrombin**. *Factor XII* - **Factor XII** (Hageman factor) is a component of the **intrinsic coagulation pathway** and is primarily synthesized in the **liver**. - It is activated upon contact with negatively charged surfaces and initiates the **intrinsic cascade**. *Fibrinogen* - **Fibrinogen** (Factor I) is a large plasma glycoprotein predominantly synthesized by **hepatocytes in the liver**. - It is converted into **fibrin** by thrombin, forming the meshwork of a stable clot.

Question 475: A 69-year-old man is given a multivitamin containing vitamin B12. He feels well, reports no symptoms, and his physical examination is normal. His complete blood count is completely normal. Which of the following statements about vitamin B12 absorption is correct?

A. absorbed primarily in the distal ileum
B. not significantly affected by folic acid deficiency
C. best absorbed in the presence of intrinsic factor (Correct Answer)
D. absorbed equally well with or without intrinsic factor

Explanation: ***best absorbed in the presence of intrinsic factor*** - **Intrinsic factor** is a glycoprotein secreted by gastric parietal cells that binds to vitamin B12, forming a complex essential for its absorption in the **terminal ileum**. - This is the **most complete answer** as it identifies the critical requirement for efficient B12 absorption. - In conditions like **pernicious anemia**, where intrinsic factor is deficient, vitamin B12 absorption is severely impaired, leading to **megaloblastic anemia** and neurological symptoms. - Only about **1-2% of oral B12** can be absorbed by passive diffusion without intrinsic factor, which is insufficient for physiological needs. *absorbed primarily in the distal ileum* - While this statement is **technically correct** (B12 is indeed absorbed in the terminal/distal ileum), it is **incomplete** as it does not mention the essential role of intrinsic factor. - The **best answer** identifies not just the location but the critical mechanism (intrinsic factor requirement). - Without specifying intrinsic factor, this option misses the most clinically important aspect of B12 absorption. *not significantly affected by folic acid deficiency* - This is **incorrect** as **vitamin B12** and **folic acid** metabolism are intricately linked in one-carbon metabolism. - Both are crucial for DNA synthesis, particularly in rapidly dividing cells like bone marrow. - Vitamin B12 (as methylcobalamin) is required to convert 5-methyl-tetrahydrofolate to tetrahydrofolate, allowing folate to participate in DNA synthesis. - Deficiency in either can lead to **megaloblastic anemia** with similar hematologic findings. *absorbed equally well with or without intrinsic factor* - This statement is **incorrect** as **intrinsic factor is essential** for efficient vitamin B12 absorption. - The B12-intrinsic factor complex binds to specific receptors (cubilin) in the terminal ileum for absorption. - Without intrinsic factor, only minimal passive diffusion occurs (~1-2%), which is inadequate to meet daily requirements of approximately 2-3 mcg.

Question 476: Tissue factor activates which coagulation pathway?

A. Common pathway
B. Contact pathway
C. Intrinsic pathway
D. Extrinsic pathway (Correct Answer)

Explanation: ***Extrinsic pathway*** - **Tissue factor (TF)**, also known as factor III, is a transmembrane glycoprotein found on extravascular cells that initiates the **extrinsic pathway** of coagulation when exposed to blood. - TF binds to **factor VIIa** to form the TF-VIIa complex, which then activates factor X and factor IX, leading to thrombin generation. *Contact pathway* - The contact pathway, also known as the **intrinsic pathway**, is initiated when factor XII comes into contact with negatively charged surfaces. - It does not involve tissue factor as its primary activator. *Intrinsic pathway* - The intrinsic pathway is initiated by activation of **factor XII** on contact with negatively charged surfaces and involves factors XI, IX, and VIII. - It works in parallel with the extrinsic pathway but is not directly activated by **tissue factor**. *Common pathway* - The **common pathway** is where the intrinsic and extrinsic pathways converge, starting with the activation of **factor X**. - While tissue factor contributes to the activation of factor X (via the extrinsic pathway), it does not directly activate or initiate the common pathway itself; its role is upstream.

Question 477: The decrease in O2 affinity of hemoglobin when the pH of blood falls is called:

A. Bohr effect (Correct Answer)
B. Haldane effect
C. Centrifugation coefficient
D. Oxygen debt

Explanation: ***Bohr effect*** - The **Bohr effect** describes the decrease in hemoglobin's affinity for oxygen in response to a drop in blood pH (increased acidity) or an increase in carbon dioxide concentration - This physiological phenomenon is crucial for efficient **oxygen delivery** to metabolizing tissues, as they produce more CO2 and H+ ions, leading to local pH decrease and facilitating oxygen release - Rightward shift of the oxygen-hemoglobin dissociation curve occurs with decreased pH *Haldane effect* - Describes the phenomenon where **deoxygenated hemoglobin** has higher affinity for CO2 and H+ ions than oxygenated hemoglobin - Important for CO2 uptake from tissues and transport to lungs - This is the reverse phenomenon - how oxygenation affects CO2 binding, not how pH affects O2 binding *Centrifugation coefficient* - A term used in biochemistry describing the **sedimentation rate** of molecules during ultracentrifugation - Has no relevance to oxygen affinity of hemoglobin or blood pH regulation - Not a physiological concept related to gas transport *Oxygen debt* - Also called **excess post-exercise oxygen consumption (EPOC)** - Refers to extra oxygen consumed after exercise to restore physiological processes - Related to exercise physiology and energy metabolism, not oxygen-binding properties of hemoglobin

Question 478: What is the major hemoglobin present in an adult?

A. HbA2
B. HbA (Correct Answer)
C. HbA1c
D. HbA1b

Explanation: ***HbA*** - **Hemoglobin A (HbA)** is the primary and most abundant form of hemoglobin found in healthy adult red blood cells, constituting about 95-98% of total hemoglobin. - It consists of two **alpha (α)** and two **beta (β)** globin chains (α2β2). *HbA2* - **Hemoglobin A2 (HbA2)** is a minor adult hemoglobin, making up only about 2-3% of total hemoglobin in adults. - It is composed of two **alpha (α)** and two **delta (δ)** globin chains (α2δ2). *HbA1c* - **Hemoglobin A1c (HbA1c)** is a glycated form of HbA, where glucose molecules are irreversibly bound to the N-terminus of the beta globin chains. - While present in adults, its proportion reflects average blood glucose levels over the past 2-3 months and is not the major form of hemoglobin. *HbA1b* - **HbA1b** is another minor glycated hemoglobin component, similar to HbA1c but present in even smaller quantities. - It is a subset of the HbA1 fraction and does not represent the major adult hemoglobin.

Question 479: Which of the following substances is primarily synthesized and secreted by mast cells?

A. Adrenaline
B. Acetylcholine
C. Heparin
D. Histamine (Correct Answer)

Explanation: ***Histamine*** - **Histamine** is a key mediator of immediate hypersensitivity reactions, primarily synthesized and stored in the **granules of mast cells** and basophils. - Upon activation, mast cells release histamine, which causes **vasodilation**, increased vascular permeability, and smooth muscle contraction, contributing to allergic symptoms. - Histamine is considered the **primary and most important mediator** released by mast cells in allergic and inflammatory responses. *Adrenaline* - **Adrenaline (epinephrine)** is a hormone and neurotransmitter primarily produced by the **adrenal medulla**. - It plays a crucial role in the **"fight or flight" response**, affecting heart rate, blood pressure, and metabolism, and is not associated with mast cell synthesis. *Acetylcholine* - **Acetylcholine** is a major neurotransmitter in the **parasympathetic nervous system** and at neuromuscular junctions. - It is synthesized and released by neurons to transmit signals, and not by mast cells. *Heparin* - **Heparin** is also synthesized and stored in mast cell granules and has anticoagulant properties. - However, while mast cells do produce heparin, **histamine** is considered the **primary and most clinically significant** substance synthesized and secreted by mast cells, particularly in the context of immediate allergic responses. - The question emphasizes "primarily," making histamine the best answer as it is the principal mediator of mast cell-related allergic reactions.

Question 480: Effect of fetal hemoglobin on the O2 dissociation curve is

A. May be right or left
B. No effect
C. Right Shift
D. Left Shift (Correct Answer)

Explanation: ***Left Shift (due to fetal hemoglobin)*** - Fetal hemoglobin (HbF) has a **higher affinity for oxygen** than adult hemoglobin (HbA) because it binds **2,3-Bisphosphoglycerate (2,3-BPG)** less avidly. - This higher affinity facilitates the **transfer of oxygen from maternal blood to fetal blood** across the placenta, causing a **left shift** in the oxygen dissociation curve. *Right Shift* - A right shift indicates a **decreased affinity for oxygen**, meaning hemoglobin unloads oxygen more readily to tissues. - This is typically seen in conditions like **acidosis**, increased **2,3-BPG**, increased **temperature**, or high **pCO2**. *No effect on fetal hemoglobin* - This statement is incorrect as fetal hemoglobin significantly impacts oxygen binding and delivery specific to the fetal environment. - The distinct structure of HbF (α2γ2) compared to HbA (α2β2) directly alters its oxygen-binding characteristics. *May be right or left* - While various factors can influence the oxygen dissociation curve in general, the *inherent property* of fetal hemoglobin itself specifically causes a **left shift**. - The primary physiological role of HbF is to enhance oxygen uptake in the low-oxygen environment of the womb, which necessitates its higher oxygen affinity.

Practice by Chapter

Composition and Functions of Blood

Practice Questions

Erythrocytes and Hemoglobin

Practice Questions

Leukocytes and Immune Function

Practice Questions

Platelets and Hemostasis

Practice Questions

Blood Groups and Transfusion

Practice Questions

Coagulation and Fibrinolysis

Practice Questions

Hematopoiesis

Practice Questions

Innate Immunity

Practice Questions

Adaptive Immunity

Practice Questions

Immunological Memory and Tolerance

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free