The sickle cell syndromes are caused by a mutation in the β-globin gene that changes the sixth amino acid from glutamic acid to valine. HbS (α 2 β 2 6 Glu→Val ) polymerizes reversibly when deoxygenated to form a gelatinous network of fibrous polymers that stiffen the RBC membrane, increase viscosity, and cause dehydration due to potassium leakage and calcium influx (Fig. 94-3). These changes also produce the sickle shape (Fig. 94-4). Sickled cells lose the pliability needed to traverse small capillaries. They possess altered “sticky” membranes that are abnormally adherent to the endothelium of small venules. These abnormalities provoke unpredictable episodes of microvascular vasoocclusion and premature RBC destruction (hemolytic anemia) in the liver and spleen. The rigid adherent cells also clog small capillaries and venules, causing tissue ischemia, acute pain, and gradual end- organ damage. This venoocclusive component usually dominates the clinical course. Prominent manifestations include episodes of ischemic pain (i.e., painful crises) and ischemic malfunction or frank infarction in the spleen, central nervous system, bones, joints, liver, kidneys, and lungs (Fig. 94-3). Several sickle syndromes occur as the result of inheritance of HbS from one parent and another hemoglobinopathy, such as β thalassemia or HbC (α 2 β 2 6 Glu→Lys ), from the other parent. The prototype disease, sickle cell anemia, is the homozygous state for HbS