Leukaemia. Light micrograph of cells from a patient suffering from lymphocytic leukaemia, a type of blood cancer. Cancerous lymphocyte cells each have a large, grey nucleus surrounded by little cell tissue (cytoplasm, pink). Lymphocytic leukaemia is caused by a genetic mutation in a lymphocyte, a white blood cell which forms part of the immune system, whilst it is in blood-forming bone marrow. This cell & its descendents multiply rapidly, reducing the marrow's ability to produce normal blood cells. There are two forms of this leukaemia: acute, producing immature cells; and chronic, producing mature-looking cells.