Anaemia. Conceptual computer artwork of three transparent erythrocytes (red blood cells) representing anaemia. Anaemia is a condition in which the blood has a reduced capacity to carry oxygen. This may be due to a reduced number of erythrocytes, or a low concentration of haemoglobin, the oxygen-carrying pigment, in the cells. There are many causes, but the most common is iron deficiency, as iron is essential for the formation of haemoglobin. Anaemia causes pallor, tiredness and weakness. Treatment is aimed at the underlying cause.