Leukemia is a malignancy (cancer) of blood cells. In leukemia, abnormal blood cells are produced in the bone marrow. Usually, leukemia involves the production of abnormal white blood cells -- the cells responsible for fighting infection. However, the abnormal cells in leukemia do not function in the same way as normal white blood cells. The leukemia cells continue to grow and divide, eventually crowding out the normal blood cells. The end result is that it becomes difficult for the body to fight infections, control bleeding, and transport oxygen.
Leukemias are further classified as myeloid or lymphoid, depending upon the type of white blood cell that makes up the leukemia cells. A basic understanding of the normal development of blood cells is needed to understand the different types of leukemia. Normal blood cells develop from stem cells that have the potential to become many cell types. Myeloid stem cells mature in the bone marrow and become immature white cells called myeloid blasts. These myeloid blasts further mature to become either red blood cells, platelets, or certain kinds of white blood cells. Lymphoid stem cells mature in the bone marrow to become lymphoid blasts. The lymphoid blasts develop further into T or B lymphocytes (T-cells or B-cells), special types of white blood cells. Myeloid or myelogenous leukemias are made up of cells that arise from myeloid cells, while lymphoid leukemias arise from lymphoid cells. Knowing the type of cell involved in leukemia is important in choosing the appropriate treatment.