Leukemia is a cancer of the blood. Leukemia begins when normal blood cells change and grow uncontrollably. Blood cells (including red cells that carry oxygen, white cells that fight infection and platelets that cause blood to clot) are produced in the bone marrow, the spongy tissue inside the larger bones in the body. Types of leukemia are named after the specific blood cell that becomes cancerous, such as the lymphoid cells (white blood cells of the immune system) or the myeloid cells (cells of the bone marrow which develop into cells which fight bacterial infections).