Hairy cell leukemia: A form of chronic leukemia in which malignant B-lymphocytes (a type of white blood cell) are seen in the bone marrow, spleen, and peripheral blood and when viewed under the microscope, these cells appear to be covered with tiny hair-like projections. Hairy cell leukemia represents 2% of all leukemia.
The characteristic features of the disease include marked enlargement of the spleen (splenomegaly), low blood cell counts (pancytopenia), a relatively small number of circulating tumor cells with a hairy appearance, and infiltration of the spleen, liver, and bone marrow by the leukemic hairy cells.
Annexin A1 (ANXA1) is a gene that is unregulated in hairy cell leukemia. ANXA1 protein expression is specific to hairy cell leukemia. Detection of ANXA1 (by immunocytochemical means) provides a simple, highly sensitive and specific assay for the diagnosis of hairy cell leukemia.
Splenectomy (surgical removal of the spleen) provides palliation (some help but not a cure). Treatment is usually with drugs, principally interferon alfa and purine analogues such as cladribine and pentostatin. For resistant cases, a promising immunotoxin has been developed that targets CD22, a molecule expressed exclusively on the surface of B-cells, including virtually all hairy cells.
Hairy cell leukemia was first described in 1958 and was misleadingly called leukemic reticuloendotheliosis.