Blood and Immunity — MCQs

Blood and Immunity Indian Medical PG Practice Questions and MCQs

Question 181: What does the CD4 count primarily refer to?

A. T helper cells (Correct Answer)
B. B cells
C. Cytotoxic cells
D. Both B and T cells

Explanation: **Explanation:** **Correct Answer: A. T helper cells** The CD4 (Cluster of Differentiation 4) molecule is a glycoprotein found primarily on the surface of **Helper T lymphocytes**. These cells play a central role in the adaptive immune system by recognizing antigens presented by MHC class II molecules on antigen-presenting cells (APCs). Once activated, they secrete cytokines that coordinate the activity of other immune cells. **Analysis of Incorrect Options:** * **B. B cells:** B cells are characterized by the presence of CD19, CD20, and CD21 markers. They do not express CD4; instead, they function in humoral immunity by producing antibodies. * **C. Cytotoxic cells:** These are T cells that express the **CD8** marker. They recognize antigens presented by MHC class I molecules and are responsible for directly killing virally infected or tumor cells. * **D. Both B and T cells:** CD4 is specific to a subset of T cells (Helper cells) and is not found on B cells. **High-Yield Clinical Pearls for NEET-PG:** * **HIV Pathogenesis:** The HIV virus specifically targets CD4+ cells by binding to the CD4 molecule via its **gp120** envelope protein. * **AIDS Definition:** A CD4 count below **200 cells/mm³** (or a CD4 percentage <14%) is a diagnostic criterion for AIDS. * **MHC Restriction Rule:** Remember the "Rule of 8": CD4 cells interact with MHC II (4 × 2 = 8), while CD8 cells interact with MHC I (8 × 1 = 8). * **Normal Range:** The typical adult CD4 count ranges from 500 to 1,500 cells/mm³.

Question 182: Which radioisotope is commonly used for measuring red cell volume?

A. Chromium (Correct Answer)
B. Cobalt
C. Iron
D. Hydrogen

Explanation: **Explanation:** The measurement of body fluid compartments relies on the **indicator dilution principle** ($Volume = \text{Amount injected} / \text{Concentration}$). To measure red cell volume specifically, a marker must bind stably to erythrocytes. **Why Chromium is Correct:** **Radioactive Chromium ($^{51}Cr$)** is the gold standard for measuring red cell volume. It binds specifically and firmly to the **beta-chain of hemoglobin**. In this procedure, a sample of the patient's blood is incubated with $^{51}Cr$ (as sodium chromate), the tagged cells are re-injected, and after equilibration, the dilution is measured to calculate the total red cell mass. **Why Other Options are Incorrect:** * **Cobalt ($^{57}Co$ or $^{58}Co$):** Primarily used in the **Schilling test** to measure Vitamin B12 absorption, not for fluid volume measurements. * **Iron ($^{59}Fe$):** While iron is central to hemoglobin, it is not used for volume measurement because it is rapidly turned over and re-utilized by the bone marrow for erythropoiesis, making it an unstable marker for dilution studies. * **Hydrogen (Tritium, $^3H$):** Tritiated water is used to measure **Total Body Water (TBW)** because it distributes uniformly across all fluid compartments. **High-Yield NEET-PG Pearls:** * **Plasma Volume:** Measured using **Radio-iodinated Serum Albumin (RISA)** or **Evans Blue dye** (T-1824). Both bind to albumin and remain in the vascular compartment. * **Extracellular Fluid (ECF) Volume:** Measured using **Inulin**, Mannitol, or Thiosulfate. * **Blood Volume Calculation:** Once red cell volume ($V_{rc}$) and Hematocrit ($Hct$) are known, Total Blood Volume = $V_{rc} / Hct$. * **$^{51}Cr$** is also used to determine the **lifespan of RBCs** in clinical studies of hemolytic anemia.

Question 183: Which of the following has the greatest effect on the ability of blood to transport oxygen?

A. Capacity of the blood to dissolve oxygen
B. Amount of hemoglobin in the blood (Correct Answer)
C. pH of plasma
D. CO2 content of red blood cells

Explanation: **Explanation:** The transport of oxygen in the blood occurs in two forms: **dissolved in plasma** and **bound to hemoglobin (Hb)**. Under normal physiological conditions, approximately **98.5%** of oxygen is transported bound to hemoglobin, while only **1.5%** is dissolved in plasma. **Why Option B is Correct:** The oxygen-carrying capacity of blood is directly proportional to the hemoglobin concentration. Each gram of hemoglobin can bind approximately **1.34 ml of oxygen** (Hüfner's constant). Because the vast majority of oxygen is carried by Hb, any change in its concentration (e.g., anemia or polycythemia) has the most significant impact on the total oxygen content of the blood compared to other factors. **Why Other Options are Incorrect:** * **Option A:** The solubility of oxygen in plasma is extremely low (0.003 ml O2/100ml blood/mmHg). Even at a high PaO2 of 100 mmHg, only 0.3 ml of O2 is dissolved, which is insufficient to meet tissue demands. * **Option C:** The pH of plasma affects the **affinity** of hemoglobin for oxygen (Bohr Effect). While a decrease in pH shifts the dissociation curve to the right (facilitating unloading), it does not change the total carrying *capacity* as significantly as the amount of Hb itself. * **Option D:** CO2 content affects oxygen binding via the carbamino effect and pH changes, but like pH, it primarily influences the loading/unloading kinetics rather than the total transport capacity. **High-Yield Clinical Pearls for NEET-PG:** * **Oxygen Content Formula:** $CaO_2 = (1.34 \times Hb \times SaO_2) + (0.003 \times PaO_2)$. This formula highlights that Hb is the primary determinant. * **Anemic Hypoxia:** In anemia, PaO2 (dissolved O2) is normal, but the total oxygen content is low because of decreased Hb. * **Carbon Monoxide (CO) Poisoning:** CO does not reduce the amount of Hb, but it reduces the *functional* Hb available for O2 transport and shifts the curve to the left.

Question 184: Most antibodies are produced in which of the following organs?

A. Heart and the liver
B. Brain and the meninges
C. Thymus and the appendix
D. Spleen and the lymph nodes (Correct Answer)

Explanation: ### Explanation **Correct Option: D (Spleen and the lymph nodes)** Antibodies (immunoglobulins) are produced by **plasma cells**, which are the terminally differentiated forms of B-lymphocytes. The production occurs primarily within the **secondary lymphoid organs**, specifically the spleen and the lymph nodes. * **Spleen:** Acts as a blood filter and is the primary site for antibody production against blood-borne antigens. * **Lymph Nodes:** Filter interstitial fluid (lymph) and serve as the major site where B-cells encounter localized antigens, undergo clonal expansion, and differentiate into antibody-secreting plasma cells. --- ### Analysis of Incorrect Options * **A. Heart and the liver:** The heart is a muscular pump with no lymphoid function. While the fetal liver is a site of hematopoiesis, the adult liver primarily produces acute-phase reactants and complement proteins, not antibodies. * **B. Brain and the meninges:** The brain is an "immunologically privileged" site protected by the blood-brain barrier. It lacks traditional lymphoid architecture for mass antibody production. * **C. Thymus and the appendix:** The **Thymus** is a *primary* lymphoid organ responsible for T-cell maturation, not antibody production. While the appendix contains lymphoid tissue (MALT), its total contribution to systemic antibody levels is negligible compared to the spleen and lymph nodes. --- ### NEET-PG High-Yield Pearls * **Primary Lymphoid Organs:** Bone marrow (B-cell maturation) and Thymus (T-cell maturation). * **Secondary Lymphoid Organs:** Spleen, Lymph nodes, Peyer’s patches, and Tonsils (Sites of antigen-antibody interaction). * **Plasma Cells:** Characterized by an eccentric nucleus and a "cartwheel" or "clock-face" chromatin appearance on histology. * **Splenectomy Risk:** Patients are at high risk for infections by **encapsulated organisms** (e.g., *S. pneumoniae, H. influenzae*) due to the loss of splenic antibody production and opsonization.

Question 185: What effect does hyperventilation have on the oxygen binding affinity of hemoglobin?

A. P50 and oxygen affinity decrease
B. P50 and oxygen affinity increase
C. P50 decreases and the oxygen affinity increases (Correct Answer)
D. P50 increases and the oxygen affinity decreases

Explanation: ### Explanation The oxygen-hemoglobin dissociation curve describes the relationship between the partial pressure of oxygen ($PO_2$) and the percentage saturation of hemoglobin. The position of this curve is determined by the **binding affinity** of hemoglobin for oxygen. **1. Why Option C is Correct:** Hyperventilation leads to the excessive "washing out" of $CO_2$ from the lungs, resulting in **hypocapnia** (decreased $PCO_2$) and **respiratory alkalosis** (increased pH). According to the **Bohr Effect**, a decrease in $H^+$ ions (alkalosis) and $PCO_2$ causes a **leftward shift** of the dissociation curve. * **Increased Affinity:** A left shift means hemoglobin holds onto oxygen more tightly. * **Decreased P50:** P50 is the $PO_2$ at which 50% of hemoglobin is saturated. When affinity increases, it takes less oxygen to saturate 50% of the heme sites; therefore, the P50 value drops. **2. Why Other Options are Incorrect:** * **Option A & B:** P50 and affinity always have an **inverse relationship**. If P50 goes down, affinity must go up, and vice versa. They cannot both increase or decrease simultaneously. * **Option D:** This describes a **rightward shift** (decreased affinity), which occurs during exercise, hyperthermia, or increased 2,3-BPG—conditions opposite to hyperventilation. **3. Clinical Pearls & High-Yield Facts:** * **Factors shifting the curve to the LEFT (Increased Affinity/Decreased P50):** Hypocapnia, Alkalosis, Hypothermia, decreased 2,3-BPG, Fetal Hemoglobin (HbF), and Carbon Monoxide poisoning (though CO prevents $O_2$ release). * **Factors shifting the curve to the RIGHT (Decreased Affinity/Increased P50):** **CADET**, face Right! (**C**O2 increase, **A**cidosis, **D**PG/2,3-BPG increase, **E**xercise, **T**emperature increase). * **Normal P50 value:** Approximately **26.7 mmHg**.

Question 186: What is the most important function of 2,3-DPG?

A. Oxygen release (Correct Answer)
B. Oxygen binding
C. Acid-base balance
D. Water-electrolyte balance

Explanation: **Explanation:** **2,3-Diphosphoglycerate (2,3-DPG)**, also known as 2,3-BPG, is a metabolic byproduct of the Rapoport-Luebering shunt in glycolysis within erythrocytes. Its primary physiological role is to act as an allosteric effector that decreases the affinity of hemoglobin (Hb) for oxygen. **Why Oxygen Release is Correct:** 2,3-DPG binds preferentially to the central cavity of deoxygenated hemoglobin (the **T-state** or Tense state). By stabilizing the T-state, it promotes the unloading of oxygen into the tissues. Without 2,3-DPG, hemoglobin would bind oxygen too tightly, preventing its release at the tissue level. An increase in 2,3-DPG shifts the **Oxygen-Dissociation Curve (ODC) to the right**, facilitating oxygen delivery. **Analysis of Incorrect Options:** * **B. Oxygen binding:** 2,3-DPG actually *inhibits* oxygen binding by stabilizing the deoxygenated form of Hb. * **C & D. Acid-base and Water-electrolyte balance:** While hemoglobin acts as a buffer and erythrocytes influence osmotic balance, 2,3-DPG does not play a direct or significant role in these regulatory processes. **High-Yield Clinical Pearls for NEET-PG:** * **Right Shift of ODC:** Remember the mnemonic **"CADET, face Right!"** (Increase in **C**O2, **A**cid/H+, **D**PG, **E**xercise, and **T**emperature all shift the curve to the right). * **Fetal Hemoglobin (HbF):** HbF has a lower affinity for 2,3-DPG compared to adult Hb (HbA) because its gamma chains lack certain binding sites. This allows HbF to have a *higher* oxygen affinity, enabling oxygen transfer from mother to fetus. * **Stored Blood:** 2,3-DPG levels decrease in stored blood. Massive transfusions of "old" blood can lead to impaired tissue oxygenation until the recipient's body regenerates 2,3-DPG. * **Adaptation:** 2,3-DPG levels increase during chronic hypoxia and at high altitudes to improve peripheral oxygen delivery.

Question 187: What is the primary site of red blood cell formation in a 20-year-old healthy male?

A. Flat bones (Correct Answer)
B. Long bones
C. Liver
D. Yolk sac

Explanation: **Explanation:** The primary site of erythropoiesis (red blood cell formation) changes dynamically throughout human development. In a **20-year-old healthy male**, the process is restricted to the **red bone marrow** found primarily in **flat bones** and the proximal ends of some long bones. **1. Why Flat Bones is Correct:** By age 20, the red marrow in the shafts (diaphyses) of long bones has been replaced by inactive, fatty yellow marrow. Active erythropoiesis is concentrated in the **flat bones** (such as the sternum, ribs, skull, and iliac crest) and the vertebrae. These sites remain the lifelong primary source of RBCs. **2. Why the Other Options are Incorrect:** * **Long bones:** While long bones are the primary site during childhood, the marrow space undergoes "fatty metamorphosis" starting around age 5. By age 20, only the proximal epiphyses of the femur and humerus retain active red marrow. * **Liver:** This is the primary site of erythropoiesis during the **second trimester** (hepatic stage) of fetal life. In adults, the liver only produces RBCs in pathological states (extramedullary hematopoiesis). * **Yolk sac:** This is the **first site** of erythropoiesis, occurring during the first few weeks of gestation (mesoblastic stage). **High-Yield NEET-PG Pearls:** * **Timeline of Erythropoiesis:** 0–2 months (Yolk sac) → 2–7 months (Liver/Spleen) → 7–9 months (Bone marrow). * **Clinical Site:** The **iliac crest** is the most common site for bone marrow aspiration/biopsy in adults because it is a rich source of red marrow. * **Extramedullary Hematopoiesis:** If the bone marrow fails (e.g., Myelofibrosis), the liver and spleen may resume RBC production, often leading to hepatosplenomegaly.

Question 188: Linzenmeyer is used to measure which of the following?

A. Bleeding time
B. Clotting time
C. Prothrombin time
D. Erythrocyte Sedimentation Rate (ESR) (Correct Answer)

Explanation: **Explanation:** The correct answer is **D. Erythrocyte Sedimentation Rate (ESR)**. **Understanding the Concept:** The Erythrocyte Sedimentation Rate (ESR) is a non-specific marker of inflammation that measures the rate at which red blood cells sink to the bottom of a column of anticoagulated blood. **Linzenmeyer’s method** is one of the classical techniques used to measure ESR. Unlike the more common Westergren or Wintrobe methods (which measure the distance cells fall in a fixed time), the Linzenmeyer method measures the **time taken** for the red cells to settle to a specific mark (usually 18 mm) in a specialized tube. **Why other options are incorrect:** * **Bleeding Time (BT):** Measured primarily by the **Duke’s method** (earlobe/finger prick) or the **Ivy’s method** (using a sphygmomanometer cuff). It assesses platelet function and capillary integrity. * **Clotting Time (CT):** Measured by the **Capillary tube method** (Wright’s) or the **Lee-White method** (venous blood in a glass tube). It assesses the intrinsic and common coagulation pathways. * **Prothrombin Time (PT):** Measured in the laboratory using **Quick’s one-stage method**. It assesses the extrinsic and common pathways (Factors VII, X, V, II, and I). **High-Yield Clinical Pearls for NEET-PG:** * **Methods of ESR measurement:** Westergren (most sensitive/standard), Wintrobe (uses a shorter tube, also used for PCV), and Linzenmeyer. * **Factors increasing ESR:** Anemia (except sickle cell), pregnancy, inflammation, malignancy, and high fibrinogen levels (which promote **Rouleaux formation**). * **Factors decreasing ESR:** Polycythemia, spherocytosis, sickle cell anemia (due to abnormal shape preventing stacking), and congestive heart failure. * **Westergren Tube:** 300 mm long, open at both ends. * **Wintrobe Tube:** 110 mm long, closed at one end.

Question 189: What is the most important source of histamine?

A. Mast cells (Correct Answer)
B. Eosinophil
C. Neutrophil
D. Macrophages

Explanation: **Explanation:** **1. Why Mast Cells are the Correct Answer:** Mast cells are the primary and most important source of histamine in the body. They are tissue-resident cells (found abundantly in the skin, lungs, and GI tract) that contain large, electron-dense cytoplasmic granules. These granules store pre-formed **histamine** bound to heparin. Upon activation—most commonly via the cross-linking of **IgE antibodies** on their surface by an allergen—mast cells undergo degranulation, releasing histamine into the local environment. This triggers vasodilation, increased vascular permeability, and smooth muscle contraction. **2. Why Other Options are Incorrect:** * **Eosinophils:** These cells are primarily involved in parasitic infections and allergic reactions. Instead of producing histamine, they contain **Histaminase**, an enzyme that breaks down histamine to limit the inflammatory response. * **Neutrophils:** These are the "first responders" of acute inflammation, primarily focused on phagocytosis and the release of lysosomal enzymes and reactive oxygen species (ROS). They do not store or secrete histamine. * **Macrophages:** These are professional antigen-presenting cells involved in phagocytosis and the secretion of cytokines (like TNF-α and IL-1), but they are not a source of histamine. **3. High-Yield Clinical Pearls for NEET-PG:** * **Basophils:** While mast cells are the primary *tissue* source, **Basophils** are the primary *circulating* (blood) source of histamine. * **Triple Response of Lewis:** Histamine release causes the classic triad of Red spot (capillary dilation), Flare (arteriolar dilation), and Wheal (exudation/edema). * **Receptors:** Remember that **H1 receptors** are involved in inflammation/allergy, while **H2 receptors** (found in gastric parietal cells) stimulate HCl secretion.

Question 190: What is the lifespan of platelets?

A. 120 days
B. 10 days (Correct Answer)
C. 30 days
D. 60 days

Explanation: **Explanation:** The correct answer is **B. 10 days**. **1. Why 10 days is correct:** Platelets (thrombocytes) are small, anucleated cell fragments derived from the cytoplasm of **megakaryocytes** in the bone marrow. Their average lifespan in the peripheral blood is approximately **7 to 10 days**. After this period, they are sequestered and destroyed by the tissue macrophage system, primarily in the **spleen**. Their short lifespan is due to the lack of a nucleus, which prevents them from synthesizing new proteins or enzymes to repair cellular damage. **2. Why the other options are incorrect:** * **A. 120 days:** This is the average lifespan of a **Red Blood Cell (RBC)**. * **C & D. 30/60 days:** These do not correspond to the physiological lifespan of any major formed element in the blood. Most white blood cells (WBCs) have lifespans ranging from hours (neutrophils) to years (memory lymphocytes), but none typically average 30 or 60 days. **3. NEET-PG High-Yield Pearls:** * **Storage:** About **1/3rd** of the total body platelets are sequestered in the **spleen** at any given time. In cases of splenomegaly, this fraction increases, leading to thrombocytopenia. * **Production:** Platelet production is regulated by **Thrombopoietin**, which is primarily synthesized in the **liver**. * **Clinical Correlation:** Because the lifespan is ~10 days, drugs that irreversibly inhibit platelets (like **Aspirin**) exert their anti-thrombotic effect for the entire life of the platelet. Therefore, aspirin must be stopped at least 7–10 days before elective surgery to allow for the generation of new, functional platelets. * **Normal Count:** 1.5 to 4.5 lakh/mm³. Critical values below 20,000/mm³ pose a high risk for spontaneous hemorrhage.

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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