The primary event in the molecular pathogenesis of SCD involves polymerization of deoxygenated HbS. Since RBCs are packaged with such a high concentration of hemoglobin (32-34 g/dL, 320-340 g/L, or 19.9-21 mmol/L), it is important that the proteins be extremely soluble.1 HbS carries oxygen normally, and when oxygenated, the solubility of HbS and HbA are the same. Once the oxygen is unloaded to the tissues HbS solubility decreases. This promotes hydrophobic interactions between the hemoglobin molecules and polymerization, which leads to the distortion of the RBC into the characteristic crescent or sickle shape.1 Polymerization of deoxy-HbS is influenced by the degree of red cell deoxygenation, MCHC, temperature, intracellular pH, intraerythrocytic HbS concentration, and intracellular HbF concentration.1
Sickled RBC Normal RBC
FIGURE 68-2. Elongated sickle-shaped and normal discoid-shaped red blood cells. (From Chan CYJ, Moore R. Sickle cell disease. In: DiPiro JT, Talbert RL, Yee GC, et al., eds. Pharmacotherapy: A Pathophysiologic Approach, 6th ed. New York: McGraw-Hill, 2005: 1856.)
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