DefinitionSickle cell anemia is a disease passed down through families in which red blood cells form an abnormal crescent shape. (Red blood cells are normally shaped like a disc.)Alternative NamesAnemia - sickle cell; Hemoglobin SS disease (Hb SS); Sickle cell diseaseCauses, incidence, and risk factorsHemoglobin is a protein inside red blood cells that carries oxygen. Sickle cell anemia is caused by an abnormal type of hemoglobin called hemoglobin S. Hemoglobin S distorts the shape of red blood cells, especially when exposed to low oxygen levels.The distorted red blood cells are shaped like crescents or sickles. These fragile, sickle-shaped cells deliver less oxygen to the body's tissues. They can also clog more easily in small blood vessels, and break into pieces that disrupt healthy bloo