Hairy cell leukemia is a rare, slow-growing cancer of the blood in which your bone marrow makes too many B cells (lymphocytes), a type of white blood cell that fights infection. These excess B cells are abnormal and look "hairy" under a microscope. As the number of leukemia cells increases, fewer healthy white blood cells, red blood cells and platelets are produced.
Hairy cell leukemia affects more men than women, and it occurs most commonly in middle-aged or older adults.
Doctors aren't sure what causes hairy cell leukemia, and there is no cure. Hairy cell leukemia is considered a chronic disease because it may never completely disappear, although treatment can lead to a remission for years.