Blood and Immunity — MCQs

Blood and Immunity Indian Medical PG Practice Questions and MCQs

Question 461: Females have a lower RBC count compared to males due to?

A. Low erythropoietin (Correct Answer)
B. High estrogen
C. Low stem cells
D. Menstrual blood loss

Explanation: ***Low erythropoietin (relative to males)*** - The primary reason females have lower RBC counts is due to **hormonal differences**, specifically the lack of androgenic stimulation of erythropoiesis that males experience. - **Testosterone in males** directly stimulates erythropoietin production and enhances erythropoiesis, leading to higher RBC counts (males: 4.5-5.5 million/µL vs females: 4.0-5.0 million/µL). - Females have relatively lower erythropoietic drive compared to males due to the absence of significant androgenic hormones, which can be conceptualized as relatively lower erythropoietic stimulus. - This difference exists across all age groups, including pre-menarchal and post-menopausal women, confirming it is **hormonal rather than blood loss-related**. *High estrogen* - Estrogen does not significantly suppress erythropoiesis to cause lower RBC counts. - Estrogen has various effects on the hematopoietic system but is not the primary cause of the gender difference in RBC count. *Low stem cells* - Hematopoietic stem cell numbers and functionality are comparable between males and females. - There is no evidence of lower stem cell counts in females accounting for RBC differences. *Menstrual blood loss* - While menstrual blood loss can contribute to **iron deficiency anemia** in some women, it does NOT explain the baseline physiological difference in RBC counts between genders. - Most healthy menstruating women maintain normal RBC counts despite regular menstruation. - The RBC count difference exists even in pre-menarchal girls and post-menopausal women, proving menstruation is not the primary cause.

Question 462: Which isotope is used to measure RBC volume?

A. Cr 51 (Correct Answer)
B. H-3
C. D2O
D. I-135

Explanation: ***Cr 51*** - **Chromium-51** attaches irreversibly to the beta chain of hemoglobin, making it an ideal tracer for measuring **red blood cell volume** and survival. - After injection, the labeled red blood cells distribute throughout the circulation, and their dilution allows for the calculation of the total **RBC mass**. *H-3* - **Tritium (H-3)** is typically used as tritiated water to measure **total body water**, as it readily equilibrates throughout all fluid compartments. - It does not specifically bind to red blood cells for mass measurement. *D2O* - **D2O (heavy water)** is used to measure **total body water** content, similar to tritiated water. - It exchanges with water in the body and diffuses into all fluid compartments, rather than targeting red blood cells. *I-135* - While various **iodine isotopes** are used in medicine, such as **I-131** for thyroid imaging or therapy, **I-135** is not a commonly used isotope for measuring red blood cell volume. - Other tracers like **radio-iodinated human serum albumin** (e.g., I-125 HSA) can be used to measure plasma volume, not specifically RBC volume.

Question 463: What is the primary physiological effect of increased 2,3-DPG on hemoglobin?

A. Increased affinity of hemoglobin to oxygen
B. Decreased affinity of hemoglobin to oxygen (Correct Answer)
C. Left shift of oxygen-hemoglobin dissociation curve
D. Right shift of oxygen-hemoglobin dissociation curve

Explanation: ***Decreased affinity of hemoglobin to oxygen*** - **2,3-Diphosphoglycerate (2,3-DPG)** binds to the beta subunits of deoxyhemoglobin, stabilizing the **deoxygenated state** and thus **reducing hemoglobin's affinity for oxygen**. - This is the **primary molecular mechanism** by which 2,3-DPG exerts its effect, facilitating **oxygen unloading** in peripheral tissues. - This decreased affinity manifests graphically as a **right shift** in the oxygen-hemoglobin dissociation curve. *Increased affinity of hemoglobin to oxygen* - This is incorrect because 2,3-DPG specifically works to **decrease hemoglobin's affinity** for oxygen, promoting oxygen release. - Increased affinity would mean oxygen is held more tightly, which is counterproductive for **oxygen delivery** to tissues. *Left shift of oxygen-hemoglobin dissociation curve* - A **left shift** indicates **increased affinity** of hemoglobin for oxygen, meaning oxygen is held more tightly. - Since 2,3-DPG decreases affinity, it causes a **right shift**, not a left shift. *Right shift of oxygen-hemoglobin dissociation curve* - While this is the **graphical representation** of 2,3-DPG's effect, it is a **consequence** of the primary molecular mechanism (decreased affinity). - A right shift signifies that for any given partial pressure of oxygen, hemoglobin is **less saturated** with oxygen, reflecting the decreased affinity caused by 2,3-DPG binding.

Question 464: In general pathophysiology, the most direct and fundamental mechanism by which death occurs typically involves which of the following?

A. Damage to the plasmalemma
B. Decreased oxygen-carrying capacity of blood
C. Poisoning of oxidative phosphorylation (Correct Answer)
D. Increased calcium transport into mitochondria

Explanation: ***Correct Option: Poisoning of oxidative phosphorylation*** - **ATP depletion** is the most fundamental and direct mechanism of cell death in pathophysiology - **Poisoning of oxidative phosphorylation** (e.g., cyanide, carbon monoxide at mitochondrial level) directly inhibits the electron transport chain, immediately stopping ATP production - Without ATP, cells cannot maintain: - **Ion gradients** (Na+/K+ ATPase failure) - **Membrane integrity** (leading to cell swelling and rupture) - **Cellular homeostasis** (all active transport mechanisms fail) - This represents the **final common pathway** of cell death - regardless of the initial insult (hypoxia, toxins, trauma), death ultimately occurs when ATP production fails - Death occurs within minutes when oxidative phosphorylation is poisoned, demonstrating its fundamental nature *Incorrect Option: Decreased oxygen-carrying capacity of blood* - While decreased oxygen-carrying capacity (severe anemia, carbon monoxide poisoning) causes **hypoxic injury**, it is one step removed from the fundamental mechanism - It causes death **by ultimately affecting oxidative phosphorylation**, making it less direct - The body has compensatory mechanisms (increased cardiac output, increased extraction) that can partially compensate for reduced oxygen-carrying capacity - This is an **upstream cause** rather than the most direct mechanism *Incorrect Option: Damage to the plasmalemma* - Plasmalemma (plasma membrane) damage is typically a **consequence** of ATP depletion, not the primary cause - When ATP fails, Na+/K+ ATPase stops working, causing cell swelling and membrane rupture - This represents a **downstream effect** of energy failure rather than the fundamental mechanism *Incorrect Option: Increased calcium transport into mitochondria* - Excessive mitochondrial calcium can trigger apoptosis and disrupt oxidative phosphorylation - However, this is a **specific mechanism** of injury in certain contexts (ischemia-reperfusion, excitotoxicity) - It is not as universally fundamental as the failure of oxidative phosphorylation itself - Calcium dysregulation is often secondary to ATP depletion and loss of calcium pump function

Question 465: Which cytokine activates eosinophils?

A. IL-1
B. IL-4
C. IL-5 (Correct Answer)
D. IL-6

Explanation: ***IL-5*** - **Interleukin-5 (IL-5)** is the primary cytokine responsible for activating **eosinophils**, stimulating their **production, maturation, and release** from the bone marrow. - It also enhances eosinophil survival and function at sites of inflammation, particularly in **allergic reactions** and **parasitic infections**. *IL-1* - **Interleukin-1 (IL-1)** is a **pro-inflammatory cytokine** primarily involved in fever, acute phase responses, and activating lymphocytes, but not directly in eosinophil activation. - It plays a role in general immune responses and inflammation by activating various cell types, including **macrophages and endothelial cells**. *IL-4* - **Interleukin-4 (IL-4)** is crucial for **Th2 differentiation** and isotype switching to IgE in B cells, promoting allergic responses. - While involved in allergic reactions, it primarily affects B cell and T cell function rather than direct activation of eosinophils, though it can support their development indirectly. *IL-6* - **Interleukin-6 (IL-6)** is a **pro-inflammatory cytokine** involved in acute phase reactions, hematopoiesis, and immune responses. - It is critical for B cell differentiation and T cell activation but does not directly activate eosinophils.

Question 466: Endothelial cells synthesize which of the following substances?

A. Fibrinogen
B. Factor-X
C. Factor-XII
D. Factor VIII (Correct Answer)

Explanation: ***Factor VIII (Correct Answer)*** * Endothelial cells synthesize and secrete **von Willebrand factor**, a carrier protein for **Factor VIII**; they also produce Factor VIII itself, playing a crucial role in hemostasis. * **Weibel-Palade bodies** within endothelial cells are storage sites for both von Willebrand factor and Factor VIII, which are released upon endothelial activation. *Incorrect: Fibrinogen* * **Fibrinogen** is primarily synthesized by the **liver** and is a key component of the coagulation cascade, forming the fibrin clot. * While endothelial cells play a role in regulating coagulation, they are not the primary site of **fibrinogen production**. *Incorrect: Factor-X* * **Factor X** (Stuart-Prower factor) is a coagulation factor that is **synthesized in the liver** and is vitamin K-dependent. * Endothelial cells do not synthesize Factor X; instead, they *regulate* its activation and activity through various mechanisms. *Incorrect: Factor-XII* * **Factor XII** (Hageman factor) is a protein of the contact activation pathway of coagulation, primarily **synthesized by the liver**. * Endothelial cells influence the activation of Factor XII on their surface but do not **produce** this coagulation factor themselves.

Question 467: The principal site for granulocytic hemopoiesis in the adult human is:

A. Red bone marrow (Correct Answer)
B. Spleen
C. Yellow bone marrow (primarily fat storage)
D. Liver

Explanation: ***Red bone marrow*** - In adult humans, **red bone marrow** is the primary site for all types of **hematopoiesis**, including granulocytic hemopoiesis. - It contains **hematopoietic stem cells** that differentiate into myeloid and lymphoid lineages, producing mature blood cells. *Liver* - The liver is a major site of **hematopoiesis during fetal development**, especially in the second trimester. - In adults, the liver only resumes hematopoietic function under conditions of **extramedullary hematopoiesis**, such as severe bone marrow failure. *Spleen* - The spleen is involved in **hematopoiesis** during fetal life and can also contribute to **extramedullary hematopoiesis** in adults when red bone marrow is compromised. - However, it is not the principal site for normal, ongoing granulocytic hemopoiesis in healthy adults. *Yellow bone marrow* - **Yellow bone marrow** primarily consists of **adipocytes (fat cells)** and has limited hematopoietic activity. - While it can convert to red bone marrow under extraordinary demand, it is not the principal site for granulocytic hemopoiesis.

Question 468: Fever occurs due to which of the following mechanisms?

A. Endorphin
B. Enkephalin
C. Histamine
D. IL-1 (Correct Answer)

Explanation: ***IL1*** - Interleukin-1 (IL-1) is a **proinflammatory cytokine** that plays a key role in the immune response and is a primary mediator of fever [1]. - It stimulates the **hypothalamus** to increase body temperature set-point, thus inducing fever. *Enkephalin* - Enkephalins are **opioid peptides** involved in pain modulation, not in the fever response. - They primarily act in the **central nervous system** and do not directly influence thermoregulation. *Endorphin* - Endorphins, like enkephalins, are also **opioid peptides** related to pain relief and mood regulation, with no significant role in fever induction. - Their main function is to provide **analgesic** effects rather than influencing body temperature. *Histamine* - Histamine is predominantly involved in **allergic responses** and inflammation but is not a direct mediator of fever. - It causes **vasodilation** and increases vascular permeability, but does not raise the body temperature set-point like IL-1 does. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Inflammation and Repair, pp. 110-111.

Question 469: Which of the following agents is least likely to be found in plasma?

A. Fibrinogen
B. Prothrombin
C. Calcium ion
D. Thrombin (Correct Answer)

Explanation: ***Thrombin*** - **Thrombin** is an active enzyme that converts **fibrinogen to fibrin** during coagulation. Its presence in significant amounts in plasma would lead to widespread, uncontrolled clotting. - Due to its potent procoagulant activity, thrombin is typically present in circulation only transiently and in very low concentrations, being rapidly inactivated by **antithrombin** and other inhibitors. *Fibrinogen* - **Fibrinogen** is a large plasma protein that is a precursor to fibrin and is crucial for **blood clot formation**. - It is a normal and abundant component of plasma, synthesized by the liver, and is continuously present in significant concentrations. *Prothrombin* - **Prothrombin** (Factor II) is the inactive precursor of thrombin, synthesized by the liver with the help of **vitamin K**. - It circulates in the plasma in relatively high concentrations, ready to be activated into thrombin upon injury. *Calcium ion* - **Calcium ions (Ca2+)** are essential **cofactors** for many steps in the coagulation cascade. - They are continuously present in plasma and play vital roles in bone health, muscle contraction, and nerve function, in addition to coagulation.

Question 470: 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.

Practice by Chapter

Composition and Functions of Blood

Practice Questions

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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