Illustration showing chimeric antigen receptor (CAR) T cells (blue) being used to treat hairy cell leukaemia (translucent with dark red nuclei). Leukaemia is a type of blood cancer in which the blood-producing tissue in bone marrow produces immature white blood cells without restraint, impairing the function of normal blood cells. Hairy-cell leukaemia is a rare form mostly affecting men. CAR-T cells are a patients own T cells, a form of white blood cell, that have been genetically modified to recognise and destroy certain cancer or autoimmune cells that might otherwise be tolerated by the immune system. Healthy red blood cells (red discs) and white blood cells (translucent/light purple) are visible in the background.