Leukaemia is cancer of the white blood cells. Acute leukaemia means it progresses rapidly and aggressively, and usually requires immediate treatment. Acute leukaemia is classified according to the type of white blood cells affected. The 2 main types of white blood cells are: lymphocytes – mostly used to fight viral infections myeloid cells – which perform a number of different functions, such as fighting bacterial infections, defending the body against parasites and preventing the spread of tissue damage