Hairy cell leukemia (HCL) is a chronic lymphoid leukemia, originally described in 1958 by Bouroncle and colleagues [1, 2] and named after the hairlike cytoplasmic projections seen on the surface of the abnormal B-cells (see the image below).
See Chronic Leukemias: 4 Cancers to Differentiate, a Critical Images slideshow, to help detect chronic leukemias and determine the specific type present.
Hairy cell leukemia is recognized as a clonal B-cell malignancy, as identified by immunoglobulin gene rearrangements that result in a phenotype B-cell expression of surface antigens. These reflect the differentiation between the immature B-cell of chronic lymphocytic leukemia and the plasma cell of multiple myeloma.
The abnormal cell in hairy cell leukemia is a clonal B-cell lymphocyte (see image below). This cell infiltrates the patient's reticuloendothelial system and interferes with bone marrow function, resulting in bone marrow failure or pancytopenia. [3, 4] The hairy cell also infiltrates the liver and spleen, resulting in organomegaly. Recently, new DNA scanning techniques using whole-exome sequencing identified 5 missense somatic clonal mutations, including a heterozygous mutation in BRAF that results in the BRAF V600E variant protein, which is oncogenic for other tumors as well, was identified in all hairy cell leukemia patients and may be responsible for the pathogenesis, diagnosis, and targeted therapy of hairy cell leukemia. 
The etiology of hairy cell leukemia has not been determined, although some investigators suggest that exposures to benzene, organophosphorus insecticides, or other solvents may be related to disease development. This hypothesis has not been confirmed by other reports, although a French study that evaluated occupational exposure to pesticides and lymphoid neoplasms among men appears to support the hypothesis that occupational pesticide exposures may not only be involved in hairy cell leukemia, Hodgkin lymphoma, and multiple myeloma, but also may play a role in non-Hodgkin lymphoma.  Further research is needed.
Exposure to radiation, agricultural chemicals, and wood dust, and a previous history of infectious mononucleosis have been suggested as etiologic associations in previous reports.
Overexpression of cyclin D1 protein, an important cell-cycle regulator, has been observed in hairy cell leukemia and may play a role in the molecular pathogenesis of the disease.
Accumulation of hairy cells in the bone marrow, liver, and spleen, with very little lymph node involvement, is characteristic of hairy cell leukemia. This pattern probably results from the expression of the integrin receptor alpha4-beta1 by the hairy cells and the interaction of the receptor with the vascular adhesion molecule-1 (VCAM-1) found in splenic and hepatic endothelia, bone marrow, and splenic stroma.
Hairy cell leukemia is relatively uncommon and accounts for 2% of all leukemia cases, which is about 600-800 new patients diagnosed each year. 
Some geographic variations have been observed with hairy cell leukemia, such as an extremely low incidence in Japan.
Abdominal discomfort is a common symptom, resulting from hepatosplenomegaly. It is usually controlled with new effective drug therapy, although late recurrences may occur. Recurrences are usually responsive to medications.
Race-, Sex-, and Age-related Demographics
Hairy cell leukemia is observed more commonly in whites. It has been reported to occur in Ashkenazi Jews, and it is rare in Japan and in those of African descent. 
The disease occurs predominantly in middle-aged men, with a median age of 52 years. The male-to-female ratio is 4:1 to 5:1. 
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