Blood and Immunity — MCQs

Blood and Immunity Indian Medical PG Practice Questions and MCQs

Question 221: What is the primary function of Kupffer cells in the liver?

A. Phagocytic (Correct Answer)
B. Lytic
C. Secretory
D. Excretory

Explanation: **Explanation:** **Kupffer cells** are specialized, stellate-shaped macrophages located within the sinusoidal lining of the liver. They are part of the **Mononuclear Phagocyte System (MPS)** (formerly known as the Reticuloendothelial System). 1. **Why Phagocytic is Correct:** The primary role of Kupffer cells is to act as the liver's first line of defense. They filter blood arriving from the portal circulation, performing **phagocytosis** to remove particulate matter, aged red blood cells, and gut-derived bacteria or endotoxins. This prevents systemic infection and maintains immunological tolerance. 2. **Why other options are incorrect:** * **Lytic:** While Kupffer cells contain lysosomal enzymes to digest engulfed material, "lytic" usually refers to cells like Natural Killer (NK) cells that destroy targets via membrane perforation (perforins). * **Secretory:** Although they can secrete cytokines (like IL-1 or TNF-α) during inflammation, this is a secondary immune response, not their primary physiological function. * **Excretory:** Excretion in the liver is primarily the function of **hepatocytes**, which produce bile to eliminate waste products like bilirubin. **High-Yield Clinical Pearls for NEET-PG:** * **Origin:** Kupffer cells are derived from circulating **monocytes**. * **Location:** They reside within the **sinusoids** (space of Disse is for Ito cells). * **Ito Cells (Stellate Cells):** Do not confuse Kupffer cells with Ito cells, which are responsible for **Vitamin A storage** and liver fibrosis (collagen production). * **Pit Cells:** These are the liver-specific Natural Killer (NK) cells.

Question 222: What is the normal reticulocyte count?

A. 0.2-2% (Correct Answer)
B. 4-6%
C. 6-10%
D. 12-14%

Explanation: **Explanation:** The **reticulocyte count** is a direct reflection of the bone marrow's erythropoietic activity. Reticulocytes are immature red blood cells that have just been released from the bone marrow into the peripheral blood. They contain residual ribosomal RNA (reticulum), which disappears within 24–48 hours as they mature into erythrocytes. **1. Why Option A is correct:** In a healthy adult, the normal reticulocyte count ranges from **0.2% to 2.0%** (some texts cite 0.5–1.5%). This low percentage indicates a steady state where the bone marrow replaces approximately 1% of the total circulating RBC mass daily to compensate for the normal destruction of aged RBCs. **2. Why other options are incorrect:** * **Options B, C, and D (4% to 14%):** These values are pathologically elevated. A count above 2% (Reticulocytosis) suggests that the bone marrow is hyperactive, usually in response to peripheral RBC loss (hemorrhage) or premature destruction (hemolysis). **Clinical Pearls for NEET-PG:** * **Reticulocyte Index (RI):** In cases of anemia, the raw percentage can be misleading. The **Corrected Reticulocyte Count** (Retic % × Patient Hct / Normal Hct) must be calculated to assess if the marrow response is adequate. * **Staining:** Reticulocytes are visualized using **Supravital stains** (e.g., New Methylene Blue or Brilliant Cresyl Blue), which cause the ribosomal RNA to precipitate into a blue-stained network. * **High-Yield Association:** An elevated reticulocyte count with a falling hemoglobin is a hallmark of **Hemolytic Anemia**. Conversely, a low reticulocyte count in the presence of anemia suggests bone marrow failure (e.g., Aplastic Anemia) or nutrient deficiencies (B12/Iron).

Question 223: The biconcave shape of RBCs is helpful because it:

A. Facilitates easy passage through capillaries. (Correct Answer)
B. Promotes rouleaux formation.
C. Increases surface area for gas exchange relative to its volume.
D. None of the above.

Explanation: ### Explanation The biconcave disc shape of the Red Blood Cell (RBC) is a specialized structural adaptation essential for its primary function: oxygen transport and microcirculatory navigation. **Why Option A is Correct:** The most critical advantage of the biconcave shape is **deformability**. Capillaries often have diameters as small as 3–5 µm, while a resting RBC is approximately 7.5 µm. The biconcave geometry provides a high **surface-area-to-volume ratio**, allowing the cell to undergo significant reversible deformation (folding or twisting) to squeeze through narrow splenic sinusoids and peripheral capillaries without rupturing the membrane. **Why Other Options are Incorrect:** * **Option B:** Rouleaux formation (stacking of RBCs) occurs primarily due to increased plasma proteins like fibrinogen or globulins (e.g., in inflammation or Multiple Myeloma). While the flat shape allows stacking, it is a pathological or incidental finding, not a physiological "benefit" of the shape. * **Option C:** While it is true that the biconcave shape increases surface area for gas exchange, this is often considered a secondary benefit. In the context of competitive exams like NEET-PG, the **mechanical flexibility** required for capillary transit is prioritized as the primary functional necessity of the shape. * **Option D:** Incorrect as Option A is the established physiological reason. **High-Yield Facts for NEET-PG:** * **Spectrin and Ankyrin:** These peripheral membrane proteins maintain the biconcave shape. * **Hereditary Spherocytosis:** A deficiency in these proteins leads to spherical RBCs. These cells lack the "excess" membrane of the biconcave shape, making them rigid and prone to hemolysis in the spleen. * **Mean Corpuscular Volume (MCV):** Normal range is 80–100 fL. * **Average RBC Life Span:** 120 days.

Question 224: Which surface protein is NOT present in the red blood cell membrane?

A. Nebulin (Correct Answer)
B. Ankyrin
C. Spectrin
D. Glycophorin

Explanation: ### Explanation The red blood cell (RBC) membrane is a complex structure composed of a lipid bilayer and a protein cytoskeleton that provides the cell with its characteristic deformability and durability. **Why Nebulin is the Correct Answer:** **Nebulin** is a large actin-binding protein found exclusively in **skeletal muscle**. It acts as a "molecular ruler" to regulate the length of thin filaments during sarcomere assembly. It is **not** a component of the RBC membrane. **Analysis of Incorrect Options:** * **Spectrin (Option C):** This is the most abundant peripheral membrane protein in RBCs. It forms a hexagonal lattice that acts as the primary "skeleton," providing structural integrity and flexibility. * **Ankyrin (Option B):** This is a key anchoring protein that attaches the spectrin lattice to the integral membrane protein, Band 3. It is crucial for stabilizing the lipid bilayer. * **Glycophorin (Option D):** This is an integral membrane protein (sialoglycoprotein). It spans the membrane and carries the negative charge (sialic acid) on the RBC surface, which prevents cells from sticking to each other and the vessel walls. **High-Yield Clinical Pearls for NEET-PG:** * **Hereditary Spherocytosis:** Most commonly caused by a deficiency in **Ankyrin** (most common) or **Spectrin**. This leads to a loss of membrane surface area, resulting in spherical, fragile RBCs. * **Hereditary Elliptocytosis:** Typically caused by defects in **Spectrin** or **Protein 4.1**. * **Vertical vs. Horizontal Interactions:** Defects in vertical interactions (Ankyrin/Band 3) lead to Spherocytosis; defects in horizontal interactions (Spectrin dimers) lead to Elliptocytosis. * **Band 3:** The most abundant integral protein; it functions as a chloride-bicarbonate exchanger (Chloride shift).

Question 225: Erythropoietin can be produced by which organ?

A. Brain
B. Kidney (Correct Answer)
C. Bone marrow
D. Adipose tissue

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 by the **peritubular interstitial cells** (fibroblast-like cells) in the renal cortex. These cells act as oxygen sensors; when they detect hypoxia (low oxygen tension), they trigger the production of Hypoxia-Inducible Factor (HIF-1α), which stimulates the EPO gene to increase hormone production. The remaining 10-15% is produced by the liver (perisinusoidal cells). 2. **Why Other Options are Incorrect:** * **Brain:** While small amounts of EPO mRNA have been detected in the brain (astrocytes) for neuroprotective roles, it is not a primary site for systemic erythropoietin production. * **Bone Marrow:** This is the **target organ** where EPO acts, not where it is produced. EPO binds to receptors on erythroid progenitor cells (CFU-E) in the bone marrow to prevent apoptosis and stimulate maturation. * **Adipose Tissue:** This tissue primarily secretes adipokines (like leptin) and is not involved in the erythropoietic pathway. **High-Yield Clinical Pearls for NEET-PG:** * **Site of Action:** EPO acts specifically on the **CFU-E (Colony Forming Unit-Erythroid)** stage. * **Fetal Life:** In the fetus, the **liver** is the primary source of EPO; the switch to the kidney occurs near birth. * **Clinical Correlation:** Chronic Kidney Disease (CKD) leads to a deficiency of EPO, resulting in **normocytic normochromic anemia**. This is treated with recombinant human erythropoietin (Epoetin alfa). * **Polycythemia:** Renal Cell Carcinoma (RCC) can sometimes produce ectopic EPO, leading to secondary polycythemia.

Question 226: Which of the following enzymes does the neutrophil use to initiate the production of toxic oxygen compounds that kill bacteria?

A. Hydrogen peroxide
B. Myeloperoxidase
C. NADPH oxidase (Correct Answer)
D. Superoxide

Explanation: ### Explanation The process by which neutrophils destroy bacteria using oxygen-dependent mechanisms is known as the **Respiratory Burst** (or Oxidative Burst). **Why NADPH Oxidase is Correct:** The "trigger" or initiating enzyme for this process is **NADPH oxidase**, located in the membrane of the phagolysosome. It catalyzes the transfer of an electron from NADPH to molecular oxygen ($O_2$), converting it into the **Superoxide anion ($O_2^-$)**. This is the critical first step in the production of Reactive Oxygen Species (ROS). **Analysis of Incorrect Options:** * **Superoxide (D):** This is the *product* of the reaction catalyzed by NADPH oxidase, not the enzyme itself. * **Hydrogen Peroxide (A):** This is a secondary metabolite formed when Superoxide undergoes spontaneous or enzymatic dismutation (via Superoxide Dismutase). It is a precursor to more potent toxins but does not initiate the burst. * **Myeloperoxidase (B):** This enzyme acts later in the pathway. It uses Hydrogen Peroxide and chloride ions to produce **Hypochlorous acid (HOCl)**, which is the active ingredient in household bleach and the most potent bactericidal agent in neutrophils. **High-Yield Clinical Pearls for NEET-PG:** * **Chronic Granulomatous Disease (CGD):** Caused by a genetic deficiency in **NADPH oxidase**. Patients suffer from recurrent infections with **catalase-positive organisms** (e.g., *S. aureus*, *Aspergillus*) because they cannot produce their own ROS. * **Nitroblue Tetrazolium (NBT) Test:** Historically used to diagnose CGD; a positive test (blue color) indicates functional NADPH oxidase. * **MPO Deficiency:** Usually asymptomatic because the preceding ROS (Superoxide and $H_2O_2$) are still produced, though the killing process is slower.

Question 227: Which of the following is a C-X-C chemokine?

A. Fractalkine
B. Lymphotactin
C. Rantes
D. IL-8 (Correct Answer)

Explanation: ### Explanation Chemokines are a family of small cytokines classified into four main groups based on the arrangement of the first two conserved **cysteine (C) residues** at the N-terminus. **1. Why IL-8 is the Correct Answer:** **IL-8 (Interleukin-8)**, also known as CXCL8, is the prototypical **C-X-C (alpha) chemokine**. In this group, the first two cysteine residues are separated by a single amino acid (X). IL-8 is primarily produced by macrophages and endothelial cells; its primary function is the **potent chemoattraction and activation of neutrophils** to sites of inflammation. **2. Analysis of Incorrect Options:** * **A. Fractalkine (CX3CL1):** This is the only member of the **C-X3-C (delta)** family. It has three amino acids separating the two cysteines. It is unique because it exists in both membrane-bound and soluble forms, acting as both an adhesion molecule and a chemoattractant. * **B. Lymphotactin (XCL1):** This belongs to the **C (gamma)** family. These chemokines lack the first and third of the four characteristic cysteines, possessing only one Cysteine residue. It is primarily chemotactic for T-cells. * **C. Rantes (CCL5):** This belongs to the **C-C (beta)** family, where the first two cysteines are adjacent. Other members include MCP-1 and MIP-1α. These generally attract monocytes, eosinophils, and lymphocytes, but *not* neutrophils. ### High-Yield Clinical Pearls for NEET-PG: * **Neutrophil Recruitment:** IL-8 is the "gold standard" marker for neutrophil chemotaxis. * **HIV Connection:** Chemokine receptors act as co-receptors for HIV entry: **CCR5** (for R5 tropic strains) and **CXCR4** (for X4 tropic strains). * **Mnemonic:** **C-C** chemokines (like Rantes) attract **C**ells other than neutrophils; **C-X-C** (like IL-8) attracts neutrophils.

Question 228: All of the following are categorized as secondary lymphoid organs except:

A. Lymph nodes
B. Spleen
C. Thymus (Correct Answer)
D. Subepithelial collections of lymphocytes

Explanation: ### Explanation The lymphoid system is divided into two functional categories: **Primary (Central)** and **Secondary (Peripheral)** lymphoid organs. **1. Why Thymus is the Correct Answer:** The **Thymus** and **Bone Marrow** are the **Primary Lymphoid Organs**. These are the sites where lymphocytes are generated (lymphopoiesis) and undergo antigen-independent maturation. In the thymus, T-cell precursors from the bone marrow differentiate into mature, immunocompetent T-lymphocytes. Since the question asks for the exception among secondary organs, the Thymus is the correct choice. **2. Analysis of Incorrect Options (Secondary Lymphoid Organs):** Secondary lymphoid organs are sites where mature lymphocytes reside, encounter antigens, and initiate an immune response (antigen-dependent activation). * **Lymph Nodes (Option A):** Filter lymph and are the primary site for neutralizing antigens from local tissues. * **Spleen (Option B):** Filters blood and mounts immune responses against blood-borne pathogens. * **Subepithelial collections (Option D):** These include **MALT** (Mucosa-Associated Lymphoid Tissue), such as Peyer’s patches in the small intestine, tonsils, and appendix. They protect mucosal surfaces. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Hassall’s Corpuscles:** Characteristic histological feature of the Thymic medulla. * **Thymic Involution:** The thymus is most active in childhood and undergoes fatty replacement (involution) after puberty. * **DiGeorge Syndrome:** Congenital failure of the 3rd and 4th pharyngeal pouches to develop, leading to thymic aplasia and T-cell deficiency. * **B-cell Maturation:** Occurs in the Bone Marrow (Primary), while their activation and proliferation into plasma cells occur in the Germinal Centers of secondary lymphoid organs.

Question 229: Which of the following factors does NOT cause a rightward shift of the oxygen-hemoglobin dissociation curve?

A. Increased hydrogen ions
B. Decreased carbon dioxide (Correct Answer)
C. Increased temperature
D. Increased 2,3-bisphosphoglycerate (BPG)

Explanation: ### Explanation The oxygen-hemoglobin (O2-Hb) dissociation curve represents the relationship between the partial pressure of oxygen ($PO_2$) and the percentage saturation of hemoglobin. A **rightward shift** indicates a **decreased affinity** of hemoglobin for oxygen, meaning oxygen is more easily released to the tissues. **Why "Decreased Carbon Dioxide" is the Correct Answer:** A decrease in $PCO_2$ (hypocapnia) causes a **leftward shift** of the curve. According to the **Bohr Effect**, lower levels of $CO_2$ (and the resulting increase in pH) increase hemoglobin's affinity for oxygen, making it bind more tightly and preventing its release. This typically occurs in the lungs. **Analysis of Incorrect Options (Factors causing a Rightward Shift):** * **Increased Hydrogen Ions (Decreased pH):** An acidic environment (acidosis) stabilizes the "Tense" (T) state of hemoglobin, promoting oxygen unloading. * **Increased Temperature:** Higher temperatures (as seen in exercising muscle or fever) weaken the bond between hemoglobin and oxygen, shifting the curve to the right to meet metabolic demands. * **Increased 2,3-BPG:** This byproduct of glycolysis binds to the beta chains of deoxyhemoglobin, stabilizing the T-state and facilitating oxygen release. Levels increase during chronic hypoxia or high-altitude adaptation. **NEET-PG High-Yield Pearls:** * **Mnemonic for Right Shift:** "**CADET**, face Right!" (**C**- $CO_2$ increase, **A**- Acidosis, **D**- 2,3-DPG/BPG increase, **E**- Exercise, **T**- Temperature increase). * **Fetal Hemoglobin (HbF):** Causes a **Left Shift** because it does not bind 2,3-BPG effectively, allowing the fetus to "pull" oxygen from maternal blood. * **P50 Value:** The $PO_2$ at which Hb is 50% saturated. A right shift **increases** the P50 (normal is ~26.7 mmHg).

Question 230: Thrombomodulin-thrombin complex prevents clotting because:

A. Thrombomodulin inhibits prothrombin activator.
B. The complex activates antithrombin III.
C. Thrombomodulin-thrombin complex activates heparin.
D. The complex removes thrombin and also activates protein kinase C, which inactivates factors V and VIII. (Correct Answer)

Explanation: **Explanation:** The correct answer is **D**. This question tests the understanding of the **Protein C Anticoagulant Pathway**, which is a vital natural mechanism to prevent excessive clot formation. **Mechanism of Action:** Thrombomodulin is a membrane-bound protein found on the surface of intact vascular endothelial cells. When **thrombin** (which is normally pro-coagulant) binds to **thrombomodulin**, it undergoes a conformational change. This complex serves two primary functions: 1. **Thrombin Removal:** It effectively "mops up" excess thrombin from the circulation, preventing it from converting fibrinogen to fibrin. 2. **Protein C Activation:** The complex activates **Protein C** (a vitamin K-dependent plasma protein). Activated Protein C (APC), along with its cofactor Protein S, proteolytically **inactivates Factors Va and VIIIa**. This shuts down the amplification of the coagulation cascade. **Analysis of Incorrect Options:** * **Option A:** Thrombomodulin does not directly inhibit the prothrombin activator (Factor Xa-Va complex); it acts downstream by inactivating the factors required for its formation. * **Option B:** Antithrombin III is a separate circulating protease inhibitor. While it inhibits thrombin, its activity is enhanced by **Heparin**, not the thrombomodulin complex. * **Option C:** Heparin is a glycosaminoglycan, not a protein activated by this complex. **NEET-PG High-Yield Pearls:** * **Factor V Leiden:** A common genetic mutation where Factor V is resistant to inactivation by Activated Protein C, leading to a hypercoagulable state (thrombophilia). * **Vitamin K Dependency:** Proteins C and S are Vitamin K-dependent. In the early stages of Warfarin therapy, these proteins decline faster than pro-clotting factors (due to shorter half-lives), which can lead to **Warfarin-induced skin necrosis**. * **Location:** Thrombomodulin is highest in the microcirculation (capillaries) to prevent clogging of small vessels.

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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