The blood disorder sickle cell anaemia is caused by a single genetic change, switching one chemical building block in part of the blood protein haemoglobin for another. This tiny alteration causes trouble for patients as it makes the haemoglobin in their red blood cells clump together, squishing the cells from a healthy doughnut-like form into a characteristic curved sickle shape. Abnormal sickle cells are stiffer than healthy ones so they get stuck in the bendy pipework of blood vessels, clogging them up and cutting off the blood supply. This image reveals damage that sickle cells have caused in the delicate network of blood vessels in the retina at the back of the eye, visualised with the fluorescent dye fluorescein. Up to 40 per cent of people with sickle cell anaemia suffer some loss of vision loss as a result of their condition, so regular monitoring is vital for saving sight.
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