Color enhanced composite of scanning electron micrographs of one normal erythrocyte or red blood cell (left) and one showing typical characteristics of sickle cell anemia (right). Sickle cell anemia is a hereditary disease of the blood in which abnormal, crescent-shaped erythrocytes are present, due to the existence of abnormal hemoglobin in the red blood cells. Sickled cells interfere with oxygen transport, obstruct capillary blood flow, and cause fever and severe pain in joints and the abdomen. A high incidence of the gene coding for sickle cell exists among Mediterranean and African populations and in African American populations in the United States.