Blood and Immunity — MCQs

Blood and Immunity Indian Medical PG Practice Questions and MCQs

Question 91: What is the primary function of lymphatic vessels?

A. To increase oncotic pressure in the blood
B. To transport proteins from interstitial fluid back to the bloodstream (Correct Answer)
C. To carry immunocompetent cells throughout the body
D. All of the above

Explanation: ### Explanation The primary physiological role of the lymphatic system is the **maintenance of fluid and protein homeostasis**. **1. Why Option B is Correct:** According to the **Starling forces** governing capillary exchange, a small amount of plasma proteins (mainly albumin) constantly leaks out of the blood capillaries into the interstitial space. These proteins cannot re-enter the high-pressure venous capillaries directly. If left in the interstitium, they would increase the interstitial oncotic pressure, leading to massive edema. Lymphatic vessels have unique "flap-valves" and high permeability, allowing them to pick up these large protein molecules and return them to the systemic circulation via the thoracic duct and right lymphatic duct. **2. Why Other Options are Incorrect:** * **Option A:** Lymphatic vessels do not increase blood oncotic pressure; rather, they preserve it by returning leaked proteins to the plasma. * **Option C:** While lymphatics do transport lymphocytes and dendritic cells, this is a secondary immune function. The *primary* hemodynamic necessity of the lymphatic system is the drainage of fluid and proteins to prevent circulatory collapse. **3. High-Yield NEET-PG Pearls:** * **Lymph Flow Rate:** Approximately **2 to 3 liters per day**. * **Chylothorax:** Obstruction or rupture of the thoracic duct leads to the accumulation of milky, triglyceride-rich lymph in the pleural space. * **Lymphedema:** Failure of lymphatic drainage (e.g., in Filariasis or post-mastectomy) leads to "pitting" edema initially, which becomes "non-pitting" over time due to fibrosis. * **Starling’s Law:** Lymph flow increases when interstitial fluid pressure rises above 0 mmHg.

Question 92: Platelet adhesion to collagen occurs via which of the following?

A. Factor VIII
B. Factor IX
C. Von-Willebrand factor (Correct Answer)
D. Fibronectin

Explanation: **Explanation:** Platelet plug formation occurs in three distinct stages: **Adhesion, Activation, and Aggregation.** **1. Why Von-Willebrand Factor (vWF) is correct:** When a blood vessel is injured, the subendothelial collagen is exposed. Platelets cannot bind directly to collagen under high-shear conditions (like in arteries). **Von-Willebrand Factor (vWF)** acts as a molecular bridge. One end of the vWF molecule binds to the exposed **collagen**, while the other end binds to the **Glycoprotein Ib-IX-V (GpIb)** receptor on the platelet surface. This tethering is the critical first step in primary hemostasis. **2. Why the other options are incorrect:** * **Factor VIII:** While vWF acts as a carrier protein for Factor VIII in the circulation to prevent its degradation, Factor VIII itself is a cofactor in the coagulation cascade (intrinsic pathway) and does not mediate platelet adhesion. * **Factor IX (Christmas Factor):** This is a serine protease in the intrinsic pathway of the coagulation cascade. Its deficiency leads to Hemophilia B. * **Fibronectin:** Although fibronectin is involved in cell adhesion and wound healing, it is not the primary mediator for the initial attachment of platelets to the subendothelium in high-flow vessels. **Clinical Pearls for NEET-PG:** * **Bernard-Soulier Syndrome:** Caused by a deficiency of the **GpIb** receptor; results in defective platelet **adhesion** and "Giant Platelets." * **Glanzmann Thrombasthenia:** Caused by a deficiency of **GpIIb/IIIa**; results in defective platelet **aggregation**. * **vWF Source:** It is synthesized and stored in **Weibel-Palade bodies** of endothelial cells and **alpha-granules** of platelets. * **Ristocetin Assay:** Used to test vWF function; ristocetin induces platelet agglutination by activating vWF to bind GpIb.

Question 93: What is the effect of sodium fluoride on the oxygen dissociation curve?

A. Right shift
B. Left shift
C. No effect on curve (Correct Answer)
D. Initial right and then left shift

Explanation: ### Explanation The correct answer is **C. No effect on curve.** **Underlying Concept:** Sodium fluoride (NaF) is a potent inhibitor of **enolase**, an enzyme in the glycolytic pathway. In red blood cells (RBCs), glycolysis is the sole source of **2,3-Bisphosphoglycerate (2,3-BPG)** via the Rappaport-Luebering shunt. When NaF is added to a blood sample, it halts glycolysis, thereby preventing the *further* synthesis of 2,3-BPG. However, 2,3-BPG is a **stability factor** for the T-state (deoxygenated) of hemoglobin. While a *decrease* in 2,3-BPG levels (as seen in stored blood) causes a left shift, the addition of NaF in a laboratory setting is intended to **preserve** the current biochemical state for glucose estimation. It does not acutely alter the existing hemoglobin-oxygen affinity or the structural configuration of the heme group required to shift the curve. **Why Incorrect Options are Wrong:** * **A. Right Shift:** A right shift occurs when hemoglobin's affinity for oxygen decreases (e.g., increased $H^+$, $CO_2$, Temperature, or 2,3-BPG). NaF does not increase any of these factors. * **B. Left Shift:** While a total absence of 2,3-BPG causes a left shift, the immediate effect of NaF is the inhibition of metabolic flux, not an immediate depletion of existing 2,3-BPG molecules already bound to hemoglobin. * **D. Initial right and then left shift:** There is no physiological mechanism by which NaF exerts a biphasic effect on hemoglobin affinity. **High-Yield Clinical Pearls for NEET-PG:** * **NaF in Lab:** It is used in "Grey-top" vacutainers for blood glucose estimation because it prevents glycolysis for up to 48–72 hours. * **2,3-BPG & Altitude:** Chronic hypoxia and high altitude lead to **increased** 2,3-BPG, causing a **Right Shift** (facilitating oxygen unloading to tissues). * **Stored Blood:** Banked blood loses 2,3-BPG over time, leading to a **Left Shift**, which means stored blood is less efficient at releasing oxygen immediately after transfusion.

Question 94: A shift of the oxygen binding curve to the left occurs because of which of the following factors?

A. Increased 2,3-DPG
B. Increased temperature
C. Increased PCO2
D. Stored blood (Correct Answer)

Explanation: ### Explanation The oxygen-hemoglobin dissociation curve represents the relationship between the partial pressure of oxygen ($PO_2$) and the percentage saturation of hemoglobin. A **left shift** indicates an increased affinity of hemoglobin for oxygen, meaning oxygen binds more tightly and is less easily released to the tissues. **Why "Stored Blood" is correct:** When blood is stored in a blood bank, the levels of **2,3-Diphosphoglycerate (2,3-DPG)** gradually decrease. 2,3-DPG is a crucial molecule that normally binds to hemoglobin and promotes the release of oxygen (right shift). In stored blood, the depletion of 2,3-DPG causes hemoglobin to hold onto oxygen more strongly, resulting in a **left shift** of the curve. **Analysis of Incorrect Options:** * **A. Increased 2,3-DPG:** This decreases hemoglobin's affinity for oxygen, shifting the curve to the **right** (facilitating oxygen unloading in tissues). * **B. Increased Temperature:** Higher temperatures (as seen in exercising muscle or fever) decrease affinity, shifting the curve to the **right**. * **C. Increased $PCO_2$:** An increase in $CO_2$ (and the resulting decrease in pH, known as the **Bohr Effect**) shifts the curve to the **right**. **Clinical Pearls for NEET-PG:** * **Left Shift (L for Love):** Hemoglobin "loves" oxygen and won't let go. Causes include: ↓ 2,3-DPG, ↓ Temp, ↓ $H^+$ (Alkalosis), ↓ $PCO_2$, and **Carbon Monoxide poisoning** (CO increases affinity of remaining sites). * **Right Shift (R for Release):** Hemoglobin "releases" oxygen to tissues. Causes include: **CADET**, face Right! (**C**$O_2$, **A**cid, **D**PG, **E**xercise, **T**emp). * **Fetal Hemoglobin (HbF):** Naturally has a higher affinity for oxygen than adult hemoglobin (HbA) to extract $O_2$ from maternal blood, thus showing a **left shift**.

Question 95: Major basic protein is secreted by which cell type?

A. Neutrophil
B. Basophil
C. Platelet
D. Eosinophil (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Eosinophil**. **Major Basic Protein (MBP)** is the primary constituent of the large crystalline core of eosinophil granules. It is a highly cytotoxic polypeptide that plays a crucial role in the body's defense against helminthic parasites. MBP acts by disrupting the cell membranes of parasites, leading to their destruction. Additionally, MBP triggers the release of histamine from mast cells and basophils, contributing to allergic inflammatory responses. **Analysis of Incorrect Options:** * **A. Neutrophil:** These cells primarily contain myeloperoxidase, lysozyme, and defensins. Their main function is phagocytosis and the respiratory burst to kill bacteria. * **B. Basophil:** These cells contain histamine, heparin, and serotonin. While they share some functional similarities with eosinophils in allergic reactions, they do not produce MBP. * **C. Platelet:** These are involved in hemostasis and contain alpha-granules (fibrinogen, vWF) and dense granules (ADP, Calcium, Serotonin), but not MBP. **High-Yield Clinical Pearls for NEET-PG:** * **Eosinophil Granule Contents:** Besides MBP, eosinophils contain **Eosinophil Cationic Protein (ECP)**, **Eosinophil-Derived Neurotoxin (EDN)**, and **Eosinophil Peroxidase (EPO)**. * **Charcot-Leyden Crystals:** These are microscopic crystals found in people with allergic diseases (like asthma) and parasitic infections; they are formed from the breakdown of **Galectin-10** (formerly thought to be MBP) within eosinophils. * **Eosinophilia:** Classically seen in **NAACP**: **N**eoplasia, **A**sthma/Allergy, **A**ddison’s disease, **C**onnective tissue disorders, and **P**arasites.

Question 96: Spontaneous bleeding occurs when platelet count falls below what level?

A. 50,000/microL
B. 40,000/microL
C. 30,000/microL
D. 20,000/microL (Correct Answer)

Explanation: **Explanation:** The correct answer is **20,000/microL**. This threshold is a critical clinical landmark in hematology. Platelets are essential for maintaining vascular integrity; they constantly "plug" minor gaps in the endothelium. When the count drops significantly (Thrombocytopenia), the risk of bleeding increases proportionally. * **Why 20,000/microL is correct:** While the normal range is 1.5–4.5 lakh/microL, the body has a significant functional reserve. Spontaneous bleeding (bleeding without trauma), such as petechiae, ecchymosis, or life-threatening intracranial hemorrhage, typically occurs only when the count falls below **20,000/microL**. Below this level, the basal requirement for endothelial maintenance is no longer met. **Analysis of Incorrect Options:** * **50,000/microL:** At this level, patients are generally asymptomatic. However, excessive bleeding may occur during **major surgery** or severe trauma. * **40,000/microL & 30,000/microL:** These levels represent moderate thrombocytopenia. While patients may bruise more easily or experience prolonged bleeding after minor injuries, *spontaneous* bleeding is rare unless there is a co-existing coagulation defect or platelet dysfunction. **High-Yield Clinical Pearls for NEET-PG:** * **Safe for Minor Surgery:** >50,000/microL. * **Safe for Major Surgery (including Neurosurgery):** >1,00,000/microL. * **Prophylactic Platelet Transfusion:** Usually indicated when the count is <10,000/microL in stable patients to prevent spontaneous bleeding. * **First Sign of Thrombocytopenia:** Petechiae (pinpoint hemorrhages) usually appearing in dependent areas like the legs.

Question 97: Which blood group is the universal donor?

A. A
B. B
C. O (Correct Answer)
D. AB

Explanation: **Explanation:** The classification of blood groups is based on the presence or absence of specific **antigens (agglutinogens)** on the surface of Red Blood Cells (RBCs) and **antibodies (agglutinins)** in the plasma. **Why Option C is Correct:** Blood group **O** is considered the universal donor because its RBCs lack both **A and B antigens**. Since there are no surface antigens for a recipient’s antibodies to attack, O-type blood can be transfused into individuals of any blood group without causing an immediate hemolytic transfusion reaction. *Note:* Specifically, **O Negative (O-)** is the true universal donor because it also lacks the Rh (D) antigen. **Why Other Options are Incorrect:** * **Option A (Group A):** These RBCs possess A-antigens. If given to a Group B or Group O recipient, their anti-A antibodies will cause hemolysis. * **Option B (Group B):** These RBCs possess B-antigens. If given to a Group A or Group O recipient, their anti-B antibodies will cause hemolysis. * **Option D (Group AB):** These RBCs possess both A and B antigens. They can only be donated to other AB individuals. However, Group AB is the **Universal Recipient** because their plasma lacks both anti-A and anti-B antibodies. **High-Yield NEET-PG Pearls:** 1. **Landsteiner’s Law:** If an antigen is present on the RBC, the corresponding antibody must be absent from the plasma (and vice versa). 2. **Bombay Blood Group (Oh):** A rare phenotype that lacks the H-antigen. They can only receive blood from another Bombay group individual, as they produce anti-H antibodies. 3. **Emergency Transfusion:** In trauma cases where blood typing is not possible, **O Negative** packed RBCs are the gold standard for immediate resuscitation.

Question 98: Platelets are derived from which of the following cell types?

A. Megakaryocytes (Correct Answer)
B. Lymphocytes
C. Neutrophils
D. Plasma cells

Explanation: **Explanation:** **Correct Answer: A. Megakaryocytes** Platelets (thrombocytes) are not whole cells but are small, discoid, anucleated fragments of cytoplasm. They are derived from **Megakaryocytes**, which are the largest cells in the bone marrow. This process, known as **thrombopoiesis**, occurs when the cytoplasm of a mature megakaryocyte forms long, branching processes called **proplatelets**. These proplatelets extend into the marrow sinusoids and fragment into individual platelets under the influence of the hormone **Thrombopoietin** (primarily produced in the liver). **Why the other options are incorrect:** * **B. Lymphocytes:** These are agranulocytic white blood cells (B-cells, T-cells, NK cells) involved in adaptive immunity. They originate from lymphoid stem cells. * **C. Neutrophils:** These are granulocytes derived from the myeloid lineage (specifically myeloblasts). They are the primary cells involved in acute inflammation and phagocytosis. * **D. Plasma cells:** These are fully differentiated B-lymphocytes that specialize in secreting large quantities of antibodies (immunoglobulins). **High-Yield Facts for NEET-PG:** * **Normal Platelet Count:** 1.5 to 4.5 lakh/mm³. * **Lifespan:** Platelets circulate for approximately **7–10 days** and are primarily sequestered/destroyed in the **spleen**. * **Key Organelle:** Platelets contain **Alpha granules** (containing clotting factors like Fibrinogen and vWF) and **Dense granules** (containing ADP, ATP, Serotonin, and Calcium). * **Clinical Correlation:** **Thrombopoietin** levels are inversely regulated by the platelet mass; low platelets lead to higher free thrombopoietin, stimulating megakaryocyte production.

Question 99: Lysozyme is present in all secretions except which of the following?

A. Sweat (Correct Answer)
B. Egg white
C. Tears
D. Saliva

Explanation: **Explanation:** **Lysozyme** (also known as muramidase) is a key component of the innate immune system. It functions as an antibacterial enzyme by hydrolyzing the β-1,4-glycosidic bonds between N-acetylmuramic acid and N-acetylglucosamine in the peptidoglycan layer of bacterial cell walls, primarily targeting Gram-positive bacteria. **Why Sweat is the Correct Answer:** Lysozyme is widely distributed in various exocrine secretions and mucosal surfaces to prevent infection. However, it is notably **absent in sweat**. The primary antimicrobial defense in sweat is not lysozyme, but rather **Dermcidin**, an antimicrobial peptide (AMP) secreted by eccrine sweat glands that exhibits broad-spectrum activity against bacteria and fungi. **Analysis of Incorrect Options:** * **Egg White:** This is one of the richest sources of lysozyme (historically called "avidin" or "G1 albumin"). It serves to protect the developing embryo from microbial invasion. * **Tears:** Secreted by the lacrimal glands, tears contain high concentrations of lysozyme to protect the cornea and conjunctiva from ocular infections. * **Saliva:** Lysozyme is a major non-antibody protective factor in saliva, working alongside lactoferrin and salivary peroxidase to maintain oral hygiene. **High-Yield NEET-PG Pearls:** * **Source:** In the blood, lysozyme is primarily derived from **neutrophils** (stored in specific/secondary granules) and **monocytes/macrophages**. * **Clinical Marker:** Elevated serum or urinary lysozyme levels are a diagnostic marker for **Acute Myelomonocytic Leukemia (AML-M4)** and **Acute Monocytic Leukemia (AML-M5)**. * **Paneth Cells:** In the gastrointestinal tract, lysozyme is secreted by Paneth cells located at the base of the Crypts of Lieberkühn.

Question 100: Which of the following is NOT a component of the dense granules of platelets?

A. ADP
B. Calcium
C. Epinephrine
D. Platelet factor 4 (Correct Answer)

Explanation: **Explanation:** Platelets contain two main types of secretory granules: **Alpha (α) granules** and **Dense (δ) granules**. Distinguishing between their contents is a high-yield topic for NEET-PG. **Why Platelet Factor 4 is the correct answer:** Platelet Factor 4 (PF4) is a component of **Alpha granules**, not dense granules. Alpha granules are the most numerous and contain large proteins involved in coagulation and vessel repair, such as Fibrinogen, von Willebrand Factor (vWF), Platelet-Derived Growth Factor (PDGF), and PF4 (which neutralizes heparin). **Analysis of Incorrect Options (Dense Granule Components):** Dense granules are smaller, less numerous, and contain non-protein molecules essential for platelet activation and vasoconstriction. They can be remembered by the mnemonic **"SAC"** or **"SADE"**: * **A. ADP (and ATP):** Crucial for recruiting and activating additional platelets via P2Y12 receptors. * **B. Calcium:** Essential for the coagulation cascade and platelet activation. * **C. Epinephrine (and Serotonin):** Act as potent vasoconstrictors and promote further platelet aggregation. **High-Yield Clinical Pearls for NEET-PG:** 1. **Storage Pool Deficiency:** A group of disorders where these granules are absent. **Hermansky-Pudlak Syndrome** is a classic example involving a deficiency of dense granules, characterized by oculocutaneous albinism and bleeding diathesis. 2. **Gray Platelet Syndrome:** A rare condition caused by a specific deficiency of **Alpha granules**, leading to large, "gray-appearing" platelets on a peripheral smear. 3. **PF4 and HIT:** Platelet Factor 4 is clinically significant in **Heparin-Induced Thrombocytopenia (HIT)**, where antibodies form against the Heparin-PF4 complex.

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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