Leukemia is different from most other cancers. Leukemia is cancer that starts in the bone marrow. The bone marrow is where new blood cells are made. Leukemia cells are early forms of blood cells. They are usually white blood cells.
When you have leukemia, your body makes too many blood cells, which are abnormal. Leukemia cells do not usually form tumors, but they can travel with the blood all over the body and reach almost any organ. Thus, leukemia can be present in many different ways, depending on which organs are involved.
Leukemia cells also interfere with the production of other blood cells — usually red blood cells and platelets.