Sickle cell anaemia. Illustration showing white blood cells (round, white), normal red blood cells (round), and red blood cells affected by sickle cell anaemia (crescent shaped). This is a 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.