Definition of Hairy cell leukemia:

Hairy cell leukemia (HCL) is a rare cancer of the blood. It affects B cells, a type of white blood cell (lymphocyte).

Causes, incidence, and risk factors:

HCL is caused by the abnormal growth of B cells. The cells can look "hairy" under the microscope because they have fine projections coming from their surface.

HCL can lead to low numbers of normal blood cells.

The cause of this disease is unknown. It affects men more often than women. The average age of onset is 55. Hairy cell leukemia is rare.