Blood and Immunity — MCQs

Blood and Immunity Indian Medical PG Practice Questions and MCQs

Question 501: The minimum platelet count required to maintain vascular integrity in the microcirculation is:

A. 10,000-20,000 (Correct Answer)
B. 5000-10000
C. 20,000-30,000
D. 1000-5000

Explanation: ***10,000-20,000/μL*** - This range represents the **critical threshold** for maintaining vascular integrity in the microcirculation. - A platelet count of **≥10,000/μL** is generally sufficient to prevent spontaneous bleeding and maintain hemostatic plugs in capillaries and small vessels. - Below 10,000/μL, the risk of **spontaneous bleeding** (petechiae, purpura) increases significantly due to inability to seal minor endothelial breaches. - This is the **standard transfusion threshold** used in clinical practice for prophylactic platelet transfusion in stable patients. *5,000-10,000/μL* - This is a **critically low range** where vascular integrity is already **compromised**. - Patients typically develop **petechiae and purpura** due to failure to maintain microvascular hemostasis. - This range indicates significant risk of spontaneous bleeding, not maintenance of integrity. *1,000-5,000/μL* - This represents **severe thrombocytopenia** with very high risk of life-threatening hemorrhage. - Spontaneous severe bleeding including **intracranial hemorrhage** and **GI bleeding** can occur. - Vascular integrity is severely compromised at this level. *20,000-30,000/μL* - While still classified as **thrombocytopenia**, this range provides a **comfortable margin of safety**. - Vascular integrity is well-maintained, and spontaneous bleeding is uncommon. - This exceeds the minimum requirement for microvascular integrity maintenance.

Question 502: Adoptive immunity is by what mechanism?

A. Vaccination with killed pathogens
B. Injection of lymphocytes (Correct Answer)
C. Natural infection
D. Passive transfer of antibodies

Explanation: ***Injection of lymphocytes*** - **Adoptive immunity** refers to the transfer of immunity by transferring immune cells, specifically **lymphocytes**, from an immune individual to a non-immune individual. - This method directly provides the recipient with pre-existing, functional immune cells capable of mediating an immune response. *Natural infection* - Natural infection leads to **active immunity**, where an individual's own immune system responds to a pathogen and generates memory cells and antibodies. - This process involves the host's immune system actively recognizing and clearing the pathogen, not the transfer of pre-formed immune cells. *Passive transfer of antibodies* - This describes **passive immunity**, where pre-formed **antibodies** are transferred from one individual to another, providing immediate but temporary protection. - While it confers immunity, it does not involve the transfer of whole immune cells (lymphocytes) that can mount a sustained cellular immune response. *Vaccination with killed pathogens* - Vaccination, even with killed pathogens, induces **active immunity** by stimulating the recipient's own immune system to produce antibodies and memory cells. - This method aims to generate a primary immune response internally rather than directly providing effector immune cells.

Question 503: The acute inflammatory response is predominantly mediated by which type of immune cells?

A. T lymphocytes
B. Neutrophils (Correct Answer)
C. Both B and T lymphocytes
D. B lymphocytes

Explanation: ***Neutrophils*** - **Neutrophils** are the **primary mediators** of the **acute inflammatory response**, being the first immune cells recruited to sites of injury or infection (usually within minutes to hours) [1], [3]. - They are **innate immune cells** that perform phagocytosis, release antimicrobial substances, and form neutrophil extracellular traps (NETs) to combat pathogens [1]. - Neutrophils constitute **50-70% of circulating leukocytes** and are the hallmark cells found in acute inflammation [3]. *T lymphocytes* - **T lymphocytes** are central to **cell-mediated immunity** in the adaptive immune response, requiring several days for activation and clonal expansion [2]. - They recognize specific antigens through TCRs and are not involved in the immediate, non-specific phase of acute inflammation. - T cells play roles in **chronic inflammation** and coordinating adaptive immunity, not acute responses. *B lymphocytes* - **B lymphocytes** mediate **humoral immunity** by producing antibodies during the adaptive immune response [1]. - Their activation, differentiation into plasma cells, and antibody production take days to weeks, making them irrelevant to the rapid acute inflammatory response. - B cells are not recruited to acute inflammatory sites in the initial phase. *Both B and T lymphocytes* - While both are critical for **adaptive immunity** and host defense, neither B nor T lymphocytes are primary mediators of acute inflammation [4]. - The acute inflammatory response relies on **innate immune cells** (neutrophils, macrophages, mast cells) for immediate, non-specific protection before adaptive immunity develops [4].

Question 504: During phagocytosis, the metabolic process called respiratory burst involves the activation of which?

A. Oxidase (Correct Answer)
B. Hydrolase
C. Dehydrogenase
D. Peroxidase

Explanation: ***Oxidase*** - During the **respiratory burst**, phagocytes activate **NADPH oxidase** (also known as phagocyte oxidase). - This enzyme catalyzes the production of **superoxide radicals** (O2-•), a highly reactive oxygen species critical for killing ingested pathogens. *Hydrolase* - **Hydrolases** are enzymes that catalyze the hydrolysis of chemical bonds, often found in lysosomes. - While lysosomal enzymes are involved in digestion within phagosomes, they are not directly responsible for the initial production of reactive oxygen species during the **respiratory burst**. *Dehydrogenase* - **Dehydrogenases** are enzymes that catalyze the removal of hydrogen atoms from a substrate, typically transferring them to an electron acceptor. - While some dehydrogenases are involved in metabolic pathways that provide precursors for the respiratory burst, they are not the enzyme directly activated to produce reactive oxygen species in the burst itself. *Peroxidase* - **Peroxidases**, such as **myeloperoxidase (MPO)**, are involved in the subsequent steps of the respiratory burst, using hydrogen peroxide (H2O2) to generate other toxic compounds like hypochlorous acid (HOCl). - However, the initial activation to produce the first reactive oxygen species (superoxide) is carried out by **NADPH oxidase**, not a peroxidase.

Question 505: Platelet adhesion is inhibited by what?

A. Nitric oxide (Correct Answer)
B. Substance P
C. IL-2
D. Thrombin

Explanation: ***Nitric oxide*** - **Nitric oxide (NO)** is a potent **vasodilator** and **inhibitor of platelet adhesion** and aggregation. - It is released by **endothelial cells** and helps maintain blood vessel patency by preventing unwanted clot formation. *Substance P* - **Substance P** is a **neuropeptide** involved in pain transmission, inflammation, and neurogenic vasodilation. - It does not directly inhibit platelet adhesion; instead, it can induce **mast cell degranulation** and promote inflammatory responses. *IL-2* - **Interleukin-2 (IL-2)** is a **cytokine** primarily involved in the activation, proliferation, and differentiation of **T lymphocytes**. - It plays a crucial role in immune responses but has no direct role in inhibiting platelet adhesion. *Thrombin* - **Thrombin** is a key enzyme in the coagulation cascade that promotes platelet activation and aggregation. - It converts **fibrinogen** to **fibrin**, leading to clot formation, thus having an opposite effect to inhibiting platelet adhesion.

Question 506: Common site of haematopoiesis in the fetus is -

A. Liver (Correct Answer)
B. Spleen
C. Bone marrow
D. Gut

Explanation: ***Liver*** - The **liver** is the **primary and most common site of hematopoiesis** during the fetal period, taking over from the yolk sac around the **second month of gestation**. - It remains the **dominant hematopoietic organ from the second to the seventh month of gestation**, making it the longest-serving primary site during fetal development. - This is why the liver is the correct answer to this question about the "common site" of fetal hematopoiesis. *Spleen* - The spleen contributes to hematopoiesis during the fetal period but is **not the primary or most common site**. - Its role is **secondary to the liver** and diminishes significantly as bone marrow becomes more active. *Bone marrow* - While it eventually becomes the **primary site of hematopoiesis in adults**, the bone marrow's role in fetal hematopoiesis becomes prominent **only in the late second and third trimesters** (after 5 months). - It gradually overtakes the liver as the main hematopoietic organ near term and postnatally. *Gut* - The **gut** is **not a site of hematopoiesis** at any stage of development. - Its primary function is digestion and absorption. **Note:** The yolk sac is the earliest site of hematopoiesis (0-2 months), but the liver serves as the primary site for the longest duration during fetal life.

Question 507: Which of the following diseases is not associated with congenital passive immunity?

A. Measles
B. Tuberculosis
C. Pertussis (Correct Answer)
D. Rubella

Explanation: ***Pertussis*** - While maternal antibodies can be passed to the fetus, the protection against **pertussis (whooping cough)** is generally **incomplete and short-lived**, making it a disease where congenital passive immunity is often insufficient. - Infants remain vulnerable to severe pertussis despite maternal antibodies, necessitating early vaccination (starting at 6 weeks of age). - This is why **maternal immunization during pregnancy** (Tdap vaccine) is now recommended to boost antibody levels. *Tuberculosis* - While maternal antibodies to *Mycobacterium tuberculosis* antigens may cross the placenta, they provide **minimal to no protective immunity** against TB. - TB immunity is predominantly **cell-mediated (T-cell based)** rather than antibody-mediated, so passive transfer of IgG offers little clinical protection. - However, **BCG vaccination at birth** can provide some protection, and maternal antibodies are generally considered to transfer, making TB traditionally included in diseases with some degree of congenital passive immunity transfer (even if not highly protective). *Rubella* - **Maternal IgG antibodies** against **rubella** virus are efficiently transferred across the placenta, conferring **strong and long-lasting passive immunity** to the newborn. - This protection typically lasts for several months, preventing rubella infection in infancy. *Measles* - Babies born to mothers immune to measles receive **maternal antibodies** that provide **excellent passive immunity** for the first 6-9 months of life. - This congenital protection is crucial for preventing measles in early infancy before the first routine measles vaccine is administered (at 9 months in India).

Question 508: When 2,3-bisphosphoglycerate (2,3-BPG) levels increase in the blood, how would this affect the oxygen-hemoglobin dissociation curve?

A. Shift to the right (Correct Answer)
B. Shift to the left
C. No change
D. Decrease in the oxygen carrying capacity

Explanation: ***Shift to the right*** - An increase in **2,3-BPG** levels causes a **rightward shift** of the oxygen-hemoglobin dissociation curve, indicating a **decreased affinity of hemoglobin for oxygen**. - This shift facilitates the **release of oxygen to tissues**, which is crucial in conditions like **hypoxia** or **anemia**. *Shift to the left* - A leftward shift would imply an **increased affinity of hemoglobin for oxygen**, hindering oxygen release to tissues. - This typically occurs with **decreased 2,3-BPG**, **alkalosis**, **hypothermia**, or presence of **fetal hemoglobin**. *No change* - **2,3-BPG** is a significant allosteric effector of hemoglobin, and its concentration directly impacts oxygen binding. - Therefore, a change in its levels *will* alter the curve, making "no change" an incorrect option. *Decrease in the oxygen carrying capacity* - A rightward shift of the curve affects hemoglobin's *affinity* for oxygen, not its **total oxygen-carrying capacity**. - Oxygen-carrying capacity is primarily determined by the **hemoglobin concentration** in the blood.

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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