Leukaemia is cancer of the white blood cells which are responsible for fighting infection. In leukaemia, the bone marrow produces abnormal levels of white blood cells. B-cell acute lymphoblastic leukaemia (B-ALL) is an aggressive (fast-growing) type of leukaemia in which too many B-cell lymphoblasts (immature white blood cells) are found in the bone marrow and blood. It is the most common type of ALL. Also called B-cell acute lymphocytic leukaemia and precursor B-lymphoblastic leukaemia.