Leg ulcer. A young man in a Third World clinic shows an ulcer on his leg due to sickle cell anaemia. This is an inherited blood disease that occurs primarily in African people and more sporadically in the Mediterranean populations. It is caused by an abnormal type of haemoglobin, known as haemoglobin S, carried by some red blood cells. The red blood cells affected have a sickle- shape, are more fragile and are easily destroyed, leading to a chronic haemolytic anaemia. The sickle cells also tend to form clots. This obstructs normal blood flow in the smaller blood vessels leading to the infarction (death) of tissues in lower and upper limbs.