Leukemia, broadly, is a cancer of the white blood cells which is characterized by rapid proliferation of these cells in the marrow. The marrow releases mostly mutant and immature cells into the blood. With relatively few healthy white blood cells, the patient's immune system eventually collapses and death follows. Modern chemotherapy can keep this disease at bay, and sometimes cure it. Marrow transplants also work, but require careful typing to ensure a compatible donation. Incompatible transplants carry serious consequences, including the painful disorder Graft vs. Host Disease.