Illustration of a red blood cell (erythrocyte) affected by sickle cell anaemia. Sickle cell anaemia is an inherited blood disease in which the red blood cells contain an abnormal form of haemoglobin (blood's oxygen-carrying pigment) that causes the blood cells to become sickle-shaped, rather than round. Sickle cells cannot move through small blood vessels as easily as normal cells and so can cause blockages. This prevents oxygen from reaching the tissues, causing severe pain and organ damage.