Acute lymphoblastic leukemia (ALL) is a cancer of the blood and bone marrow. Lymphocytes are a type of white blood cell. With ALL, the bone marrow makes too many of these cells. ALL begins in immature lymphocytes. It progresses very quickly. It may occur at any age, but is most common in children. It is also common as the end stage of a chronic leukemia of adults called chronic lymphocytic leukemia . Childhood ALL was one of the first cancers for which effective chemotherapy was developed. ALL remains highly curable today.
Cancer occurs when cells in the body become abnormal. They divide without control or order. Leukemia is cancer of the white blood cells and their parent cells. Leukemia cells do not function normally. They cannot do what normal blood cells do. In this case they can not fight infections. This means that the person is more likely to become infected with viruses or bacteria. The cancerous cells also overgrow the bone marrow. This forces other normal components, like platelets out. Platelets are needed to help the blood clot. As a result, people with leukemia may bleed more easily.