What Hb variant is implicated in sickle cell disease and what triggers sickling?

What Hb variant is implicated in sickle cell disease and what triggers sickling?

• A. Hemoglobin A; triggers include high oxygen
• B. Hemoglobin C; triggers include high pH
• C. Hemoglobin S; triggers include low oxygen tension, dehydration, acidosis, and stressors
• D. Hemoglobin F; triggers include cold exposure

Answer: (C)

Sickle cell disease arises from a variant of hemoglobin that sticks together when it has less oxygen, forming long fibers that distort red blood cells into a sickle shape. The culprit is hemoglobin S, created by a mutation in the beta-globin chain. Under low oxygen tension, hemoglobin S polymerizes, causing the cells to become rigid and crescent-shaped, which leads to vaso-occlusion and hemolysis. Triggers that promote this process include environments or conditions that lower blood oxygen or increase the concentration of hemoglobin S. Low oxygen tension directly promotes deoxygenated S, dehydration concentrates HbS in the blood and raises the likelihood of polymer formation, and acidosis shifts the balance toward the deoxygenated form, facilitating polymerization. Stressors like infection, fever, and physical exertion also raise the risk by increasing metabolic demand and promoting periods of tissue hypoxia. Other hemoglobins mentioned—normal adult hemoglobin A, the variant HbC, and fetal HbF—do not cause sickling in the same way. HbA is the normal form that does not polymerize; HbC causes a different, typically milder RBC abnormality; HbF inhibits sickling by reducing HbS polymerization, which is why higher HbF levels lessen disease severity.

Sickle cell disease arises from a variant of hemoglobin that sticks together when it has less oxygen, forming long fibers that distort red blood cells into a sickle shape. The culprit is hemoglobin S, created by a mutation in the beta-globin chain. Under low oxygen tension, hemoglobin S polymerizes, causing the cells to become rigid and crescent-shaped, which leads to vaso-occlusion and hemolysis.

Triggers that promote this process include environments or conditions that lower blood oxygen or increase the concentration of hemoglobin S. Low oxygen tension directly promotes deoxygenated S, dehydration concentrates HbS in the blood and raises the likelihood of polymer formation, and acidosis shifts the balance toward the deoxygenated form, facilitating polymerization. Stressors like infection, fever, and physical exertion also raise the risk by increasing metabolic demand and promoting periods of tissue hypoxia.

Other hemoglobins mentioned—normal adult hemoglobin A, the variant HbC, and fetal HbF—do not cause sickling in the same way. HbA is the normal form that does not polymerize; HbC causes a different, typically milder RBC abnormality; HbF inhibits sickling by reducing HbS polymerization, which is why higher HbF levels lessen disease severity.