Acute leukaemia. Light micrograph of blood cells from bone marrow in a case of acute leukaemia. Leukaemia is a cancer where certain blood cells form in excess. There are several acute forms, classified according to the type of cell affected. This non-classified form is midway between the lymphoblastic and myeloblastic forms of acute leukaemia. Lymphoblasts and myeloblasts (the large red cells) are both precursor cells to types of white blood cells. The pale orange cells are red blood cells.