2. MedlinePlus Medical Encyclopedia: Hairy cell leukemia
HCL is caused by the abnormal growth of B cells. It affects B cells, a type of white blood cell (lymphocyte) Causes. HCL can lead to low numbers of normal blood cells. Hairy cell leukemia (HCL) is a rare cancer of the blood.
http://www.nlm.nih.gov/medlineplus/ency/article/000592.htm

3. Leukemia
In leukemia, abnormal white blood cells divide out of control and crowd out the normal cells in the bloodstream. The abnormal white blood cells are not mature, and therefore cannot carry out their infection-fighting function in the blood.
http://www.acor.org/ped-onc/diseases/leuk.html

4. CNN.com - Health: Condition Clinic
In most types of leukemia, the abnormal cells are white blood cells. When leukemia develops, the body produces large numbers of abnormal blood cells. In acute leukemia, the abnormal blood cells are blasts that remain immature and cannot carry out...
http://www.cnn.com/HEALTH/condition.clinic/leukemia.html