Chemotherapy is often used to treat leukemia. Leukemia is cancer of the blood cells. Most blood cells form in the bone marrow. In leukemia, cancerous blood cells form and crowd out the healthy blood cells in the bone marrow.he type of leukemia depends on the type of blood cell that has become cancerous.For example, acute lymphoblastic leukemia is a cancer of the lymphoblasts (white blood cells that fight infection).hemotherapy is often used to treat leukemia. Leukemia is cancer of the blood cells. Most blood cells form in the bone marrow. In leukemia, cancerous blood cells form and crowd out the healthy blood cells in the bone marrow. The type of leukemia depends on the type of blood cell that has become cancerous.