Leukemia causes abnormal forms of these cells to proliferate uncontrollably and to build up in the blood and bone marrow, overwhelming healthy blood cells and incapacitating the immune system. These are four principal types of leukemia. They are classified according to the type of blood cell affected and the rate at which the disease progresses. Lymphocytic leukemia is cancer of the lymphocytes, immune cells that launch highly-targeted attacks against foreign pathogens.
Myelogenous leukemia refers to cancer of blood cells other than lymphocytes.
Each type has its own target group. It is twice as common in men as in women, affecting most people over the age of 55. In the adult population, leukemia is evenly split between acute and chronic forms. Overall, slightly more men than women suffer from leukemia. They resemble the symptoms of many other illnesses, especially in children.