Blood and Immunity — MCQs

Blood and Immunity Indian Medical PG Practice Questions and MCQs

Question 421: Which factor has the most significant influence on the oxygen dissociation curve?

A. 2,3-BPG (Correct Answer)
B. pH
C. Temperature
D. All of these

Explanation: ***2,3-BPG*** - **2,3-bisphosphoglycerate (2,3-BPG)** is a metabolic intermediate produced specifically in red blood cells that serves as the primary physiological regulator of hemoglobin's oxygen affinity. - It binds to the central cavity of deoxygenated hemoglobin, stabilizing the tense (T) state and significantly decreasing oxygen affinity, shifting the curve to the right. - Its concentration increases in chronic hypoxic conditions (high altitude, anemia, chronic lung disease), providing sustained adaptation for oxygen delivery to tissues. - **2,3-BPG levels can increase by 50% or more** during chronic hypoxia, representing the most significant **long-term physiological mechanism** for modulating the oxygen dissociation curve. *pH* - A decrease in **pH** (Bohr effect) shifts the oxygen dissociation curve to the right by stabilizing the T state of hemoglobin. - This is primarily an **acute response** to metabolic conditions rather than a sustained regulatory mechanism. - While clinically important, pH changes are typically secondary to metabolic states rather than a primary regulatory mechanism. *Temperature* - An increase in **temperature** causes a rightward shift of the oxygen dissociation curve, promoting oxygen release from hemoglobin. - Temperature effects are generally **passive responses** to environmental or metabolic conditions rather than active regulatory mechanisms. - The magnitude of temperature-induced shifts is typically smaller than those produced by 2,3-BPG in physiological conditions. *All of these* - While pH, temperature, and 2,3-BPG all influence the oxygen dissociation curve, the question asks for the factor with the **most significant influence**. - **2,3-BPG** is unique as the only factor that represents an **active, sustained, physiological regulatory mechanism** specifically evolved for oxygen delivery modulation. - pH and temperature effects are important but represent **passive responses** to metabolic conditions rather than primary regulatory control mechanisms.

Question 422: Which factor predominantly influences the rightward shift of the oxygen dissociation curve?

A. pH (Bohr effect)
B. 2,3-Bisphosphoglycerate (2,3-BPG) (Correct Answer)
C. Temperature increase
D. Carbon monoxide levels

Explanation: ***2,3-Bisphosphoglycerate (2,3-BPG)*** - **2,3-BPG** is an organic phosphate found in **red blood cells** that serves as the **predominant regulator** of oxygen-hemoglobin affinity under physiological conditions. - An increase in **2,3-BPG** levels binds to the **beta chains of deoxyhemoglobin**, stabilizing the T (tense) state and reducing hemoglobin's affinity for oxygen, thereby shifting the curve to the right and facilitating **oxygen release** to tissues. - **2,3-BPG** is especially important in **chronic adaptations** to hypoxia (high altitude, chronic lung disease, anemia) and is the **primary mechanism** for sustained alterations in oxygen delivery. - Normal RBC 2,3-BPG concentration is approximately equal to hemoglobin concentration, making it a **quantitatively significant** regulatory factor. *pH (Bohr effect)* - A decrease in blood **pH** (increased acidity) due to higher **CO2** and **H+** concentrations also shifts the oxygen dissociation curve to the right via the **Bohr effect**. - While physiologically important for **acute regulation** in metabolically active tissues, the Bohr effect operates in conjunction with other factors rather than as the predominant standalone regulator. - The effect is mediated by **protonation of histidine residues** on hemoglobin, causing conformational changes that reduce oxygen affinity. *Temperature increase* - An increase in **temperature** reduces hemoglobin's affinity for oxygen, shifting the oxygen dissociation curve to the right. - This effect is vital for **oxygen delivery** to actively metabolizing tissues (which generate heat), but is generally a **secondary factor** compared to 2,3-BPG in terms of overall regulation. - The temperature effect is more situational, occurring primarily in tissues with elevated metabolic activity. *Carbon monoxide levels* - **Carbon monoxide (CO)** causes a **leftward shift** of the oxygen dissociation curve, not a rightward shift. - CO binds to hemoglobin with 200-250 times greater affinity than oxygen, forming **carboxyhemoglobin** (COHb). - This not only reduces oxygen-carrying capacity but also **increases hemoglobin's affinity** for the remaining oxygen, making it harder to release oxygen to tissues. - CO poisoning is therefore dangerous both because it displaces oxygen and because it impairs oxygen delivery through leftward shift.

Question 423: What is the mechanism by which hemolysis causes jaundice?

A. Increased production of conjugated bilirubin due to liver dysfunction
B. Decreased conjugation of bilirubin in the liver
C. Increased reabsorption of bilirubin in the intestines
D. Increased production of unconjugated bilirubin due to hemolysis (Correct Answer)

Explanation: ***Increased production of unconjugated bilirubin due to hemolysis*** - Hemolysis leads to the **destruction of red blood cells**, releasing a large amount of **hemoglobin**. - Hemoglobin is then broken down into **heme**, which is converted into **unconjugated bilirubin** at a rate that exceeds the liver's capacity to conjugate it, leading to its accumulation. *Increased production of conjugated bilirubin due to liver dysfunction* - This scenario describes **intrahepatic or post-hepatic jaundice**, where the liver either cannot excrete conjugated bilirubin or there is an obstruction to bile flow. - In hemolysis, the liver's conjugating capacity may be overwhelmed, but the primary issue is the overproduction of **unconjugated bilirubin**, not conjugated bilirubin due to liver dysfunction. *Decreased conjugation of bilirubin in the liver* - This mechanism is characteristic of conditions like **Gilbert's syndrome** or **Crigler-Najjar syndrome**, where specific enzyme deficiencies impair the liver's ability to conjugate bilirubin. - While it results in increased unconjugated bilirubin, in hemolysis, the liver's conjugating enzymes are generally functional but simply overwhelmed by the sheer volume of bilirubin produced. *Increased reabsorption of bilirubin in the intestines* - This process, known as the **enterohepatic circulation of bilirubin**, involves the reabsorption of unconjugated bilirubin from the gut. - While it contributes to overall bilirubin levels, it is not the primary mechanism by which massive red blood cell destruction directly causes jaundice in hemolysis.

Question 424: A 45-year-old patient with chronic anemia presents with fatigue, dyspnea, and visible cyanosis. Recent blood tests show hemoglobin of 7 g/dL. Which of the following best explains the presence of cyanosis in this patient?

A. Methemoglobinemia
B. Increased deoxyhemoglobin concentration (Correct Answer)
C. Peripheral vasoconstriction
D. Decreased oxygen-carrying capacity of blood

Explanation: ***Increased deoxyhemoglobin concentration*** - Cyanosis is defined as visible bluish discoloration of skin and mucous membranes that occurs when **≥5 g/dL of deoxygenated hemoglobin** is present in capillary blood. - By definition, cyanosis requires a sufficient **absolute amount of deoxyhemoglobin**, not just a low total hemoglobin. - In this patient with Hb 7 g/dL, if cyanosis is indeed present, it indicates that despite the anemia, there is still sufficient deoxyhemoglobin concentration to cross the threshold for visible cyanosis, likely due to severe hypoxemia or additional cardiopulmonary pathology. - **Important note:** Severe anemia alone typically does NOT cause cyanosis (patients appear pale); the presence of cyanosis here suggests a coexisting condition causing increased deoxyhemoglobin. *Decreased oxygen-carrying capacity of blood* - While this accurately describes the pathophysiology of anemia (reduced total hemoglobin), it does **not explain cyanosis**. - In fact, decreased oxygen-carrying capacity in severe anemia typically **prevents cyanosis** because there is insufficient total hemoglobin to generate the 5 g/dL deoxyhemoglobin threshold needed for visible cyanosis. - Classic teaching: "Anemic patients do not become cyanotic; they become pale." *Methemoglobinemia* - This condition involves hemoglobin oxidized to the ferric state (Fe³⁺), which cannot bind oxygen and causes a characteristic slate-gray or brownish cyanosis. - While methemoglobinemia can coexist with anemia, there is no indication in the clinical scenario (no exposure history, no "chocolate-brown" blood description) to suggest this diagnosis. *Peripheral vasoconstriction* - Peripheral vasoconstriction reduces blood flow to the skin, causing **pallor or a mottled appearance**, not true cyanosis. - Cyanosis is specifically due to increased deoxyhemoglobin in capillary blood, not reduced perfusion alone.

Question 425: What role do lymphocytes have in the body?

A. Blood clotting
B. Immune response (Correct Answer)
C. Oxygen transport
D. Nutrient absorption

Explanation: ***Immune response*** - Lymphocytes, including **T cells** and **B cells**, are central to the **adaptive immune system**, recognizing and targeting specific pathogens and abnormal cells. - They produce **antibodies** (B cells) and mount **cell-mediated responses** (T cells) to protect the body from infections and cancer. *Blood clotting* - **Platelets** and **coagulation factors** are primarily responsible for blood clotting (hemostasis). - Lymphocytes do not directly participate in the formation of blood clots. *Oxygen transport* - **Red blood cells (erythrocytes)**, specifically their **hemoglobin** content, are responsible for transporting oxygen throughout the body. - Lymphocytes play no role in oxygen carriage. *Nutrient absorption* - **Enterocytes** in the small intestine are the primary cells responsible for nutrient absorption. - The lymphatic system (which includes lymphocytes) plays a role in absorbing **dietary fats**, but lymphocytes themselves are not directly involved in the absorption process.

Question 426: Which of the following is the earliest change to occur in response to chronic hypoxia?

A. Increased blood pH
B. Increased hematocrit
C. Increased erythropoietin production
D. Increased 2,3-BPG levels (Correct Answer)

Explanation: ***Increased 2,3-BPG levels*** - Increased **2,3-bisphosphoglycerate** (2,3-BPG) levels in red blood cells is the **earliest compensatory mechanism** to improve tissue oxygen delivery in chronic hypoxia. - It begins to increase within **hours** and shifts the **oxygen-hemoglobin dissociation curve to the right**, reducing hemoglobin's affinity for oxygen and facilitating oxygen release to tissues. - This represents the first **metabolic adaptation** specifically targeting oxygen unloading at the tissue level. *Increased hematocrit* - An increased hematocrit, or **polycythemia**, is a **later compensatory response** to chronic hypoxia, mediated by erythropoietin. - It takes **several days to weeks** to develop as it requires new red blood cell production from the bone marrow. *Increased blood pH* - **Chronic hypoxia** leads to **hyperventilation** and **respiratory alkalosis**, which increases blood pH **immediately** (within minutes). - However, this is a **ventilatory response** to hypoxia, not a specific adaptation for improved oxygen delivery to tissues. - In fact, alkalosis **increases** hemoglobin's oxygen affinity (left shift), which is counterproductive for tissue oxygen delivery. *Increased erythropoietin production* - **Erythropoietin (EPO)** production in the kidneys is stimulated by hypoxia within hours, leading to increased red blood cell production. - While EPO levels rise relatively early, the **actual increase in red blood cell mass** (increased hematocrit) takes days to weeks, making this a delayed compensatory mechanism. - 2,3-BPG acts faster than the erythropoietic response can produce functional changes in oxygen-carrying capacity.

Question 427: A rightward shift in the oxygen-hemoglobin dissociation curve is caused by which of the following factors?

A. Decreased temperature
B. Increased pH
C. Decreased CO2 levels
D. Increased 2,3-BPG levels (Correct Answer)

Explanation: ***Increased 2,3-BPG levels*** - **2,3-Bisphosphoglycerate (2,3-BPG)** binds to the deoxyhemoglobin allosterically, stabilizing the **T (tense) state**, which has a lower affinity for oxygen. - This stabilization favors the release of oxygen to the tissues, thus shifting the oxygen-hemoglobin dissociation curve to the **right**. *Decreased temperature* - A **decrease in temperature** stabilizes the **R (relaxed) state** of hemoglobin, increasing its affinity for oxygen. - This effect shifts the oxygen-hemoglobin dissociation curve to the **left**, promoting oxygen uptake in the lungs. *Increased pH* - An **increase in pH** (alkalosis) also stabilizes the **R state** of hemoglobin, enhancing its affinity for oxygen. - This is known as the **Bohr effect**, and it results in a **leftward shift** of the curve. *Decreased CO2 levels* - **Decreased CO2 levels** lead to a decrease in H+ concentration, increasing pH and stabilizing the **R state** of hemoglobin. - This results in an **increased affinity for oxygen** and a **leftward shift** in the oxygen-hemoglobin dissociation curve.

Question 428: What is the primary function of erythrocytes in the human body?

A. Blood clotting
B. Defense against pathogens
C. Transport of oxygen (Correct Answer)
D. Nutrient transport

Explanation: ***Transport of oxygen*** - Erythrocytes, also known as **red blood cells**, contain **hemoglobin**, a protein specialized in **binding and transporting oxygen** from the lungs to the body's tissues. - This function is vital for **cellular respiration** and the overall metabolic processes of the body. *Blood clotting* - **Platelets** (thrombocytes) are primarily responsible for **blood coagulation** and stopping bleeding by forming a clot at the site of injury. - Erythrocytes do not play a direct role in the **clotting cascade**. *Defense against pathogens* - **Leukocytes** (white blood cells), such as neutrophils, macrophages, and lymphocytes, are the primary cells involved in the **immune response** and defense against pathogens. - While essential for survival, erythrocytes are not part of the **immune system**. *Nutrient transport* - While blood plasma transports nutrients, erythrocytes themselves are **not directly responsible** for their transport. - **Plasma** carries dissolved nutrients like glucose, amino acids, vitamins, and minerals throughout the body.

Question 429: Which of the following best describes the function of hemoglobin in the blood?

A. Transport of oxygen (Correct Answer)
B. Buffering blood pH
C. Clot formation
D. Immune response

Explanation: ***Transport of oxygen*** - **Hemoglobin** is a **metalloprotein** in red blood cells crucial for binding and transporting **oxygen** from the lungs to tissues. - Its structure, containing **heme groups** with iron, allows for efficient reversible binding of oxygen molecules. *Buffering blood pH* - While hemoglobin does have some minor buffering capacity, contributing to a stable **blood pH**, it is not its primary function. - The main systems for pH regulation are the **bicarbonate buffer system** and renal mechanisms. *Clot formation* - **Clot formation** is primarily mediated by **platelets** and a complex cascade of **coagulation factors** in the plasma. - Hemoglobin is not directly involved in the process of hemostasis or clot formation. *Immune response* - The **immune response** is primarily carried out by **white blood cells (leukocytes)**, antibodies, and the complement system. - Hemoglobin does not play a role in recognizing or fighting pathogens.

Question 430: What is the primary function of neutrophils in the immune response?

A. Antibody production
B. Phagocytosis of pathogens (Correct Answer)
C. Killing infected host cells
D. Presenting antigens

Explanation: ***Phagocytosis of pathogens*** - Neutrophils are the **most abundant type of white blood cell** and are rapid responders to sites of infection. - Their primary role is to **engulf and digest (phagocytose)** bacteria, fungi, and other foreign particles using powerful enzymes and reactive oxygen species. *Antibody production* - **B lymphocytes (plasma cells)** are responsible for producing antibodies, which are proteins that recognize and neutralize specific pathogens. - Neutrophils do not produce antibodies; they are part of the **innate, non-specific immune system**. *Killing infected host cells* - **Cytotoxic T lymphocytes (CTLs)** and **Natural Killer (NK) cells** are primarily responsible for identifying and killing host cells that have been infected by viruses or have become cancerous. - While neutrophils can kill certain pathogens, their role is not to directly eliminate infected host cells. *Presenting antigens* - **Antigen-presenting cells (APCs)**, such as macrophages, dendritic cells, and B cells, are specialized to process and present antigens to T lymphocytes. - Neutrophils are not considered primary APCs and have a limited role in initiating adaptive immune responses.

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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