Sickle cell anemia, an hereditary hemoglobinopathy, results from an amino acid substitution in the beta chain of the globin portion of the hemoglobin molecule. Under decreased oxygen tension such molecules undergo structural rearrangement with the formation of needle-like "tactoids" which distort the red cell from a biconcave disk to a "sickle" shape. The consequences include a hemolytic anemia and a tendency for the sickle cells to sludge in small blood vessels with resultant tissue hypoxia and necrosis.Manifestations usually do not appear until after four to six months of life. The chronic hemolytic anemia may give rise to listlessness, irritability, jaundice, and splenomegaly. At unpredictable intervals, there are various vasocclusive crises. These most commonly involve the bones and soft tissues resulting in attacks of pain and swelling around joints. In the young infant there is a predilection for involvement of the hands and feet (the "hand-foot syndrome").