Sickle cell disease (also known as sickle cell anemia) is hereditary anemia that occurs because of an abnormality in hemoglobin. Hemoglobin is the substance in red blood cells that carries oxygen. Because of the abnormal hemoglobin, red blood cells become sickle-shaped instead of round and carry too little oxygen. When these sickled red blood cells clump together inside small blood vessels, they block blood supply to body tissues. This causes severe pain, as well as short-term or long-term damage, in almost any affected body part. Cells also get torn apart, causing severe anemia (low red blood cell count). Infection, stress, exercise, dehydration, high altitude, temperature changes, or anesthesia can trigger blood cells to sickle and lead to a painful “crisis.”