Leukemia is a cancer of the body’s blood-forming tissues, including the bone marrow and the lymphatic system. It is a progressive disease in which the bone marrow and other blood-forming organs produce increased numbers of immature, or abnormal, white cells. The white cells suppress the production of normal blood cells, leading to anemia and other symptoms that can be life threatening. Some forms of leukemia are more common in children; others occur mainly in adults.