Hairy cell leukemia is an indolent, chronic B-cell lymphoproliferative disorder comprising approximately 2 to 3% of all adult leukemias in the United States. Hairy cells are clonal expansions of mature, activated B-cells. They co-express CD11c, CD19, CD20, CD22, CD25, and CD103. Hairy cells possess clonal immunoglobulin gene rearrangements and express monoclonal surface immunoglobulin of either IgG or multiple heavy-chain isotypes. Treatment of hairy cell leukemia should be considered for symptomatic patients. It is indicated in patients with significant neutropenia, anemia, thrombocytopenia, symptomatic splenomegaly, constitutional symptoms due to hairy cell leukemia, or recurrent serious infections. Many treatments exist, including cladribine, pentostatin, interferon-alpha, splenectomy, rituximab (mabthera), and BL-22 immunotoxin.
Abbreviations:ADA adenosine deaminase, HCL hairy cell leukemia, TRAP tartrate-resistant acid phosphatase
Hairy cell leukemia (HCL) is an indolent, chronic, B-cell lymphoproliferative disorder involving the bone marrow and spleen. Reactive marrow fibrosis and peripheral blood cytopenias are commonly found.
It was initially recognized in 1923 by Ewald  who described it as leukämische reticuloendotheliose. The first detailed description was by Bouroncle et al.  in 1958, who referred to it as leukemic reticuloendotheliosis. In 1966, Schrek and Donnelly , were the first to use the descriptive term hairy cell leukemia, highlighting the irregular cytoplasmic projections of the abnormal mononuclear cells observed in the blood and marrow.
HCL is an uncommon disease, accounting for approximately 2 to 3% of all adult leukemias in the United States, which translates into approximately 600 new patients diagnosed in the United States annually. HCL tends to be seen in middle-aged Caucasian males. There is a 4:1 male predominance and a higher incidence in males of Ashkenazi Jewish heritage. Familial cases have been described [4–6]. It is rare in persons of African or Asian descent. The median age at presentation is 52 years, and it has not been described in children or teenagers.