Hairy cell leukemia is another chronic lymphoproliferative disorder that is now known to be a clinical entity distinct from CLL. Similar to CLL, it is a proliferation of neoplastic B lymphocytes. Hairy cell leukemia cells also expresses the receptor for interleukin 2 known as CD-25; this suggests that the arrest in differentiation for these lymphocytes occurs later than that for CLL but earlier than that of the lymphocytes in myeloma.

Most patients with hairy cell leukemia present with pancytopenia. Circulating hairy cells are seen in approximately 90% of the cases. The typical hairy cell is a large lymphocyte with an eccentric nucleus, and the cytoplasm typically has fine irregular projections. The tartaric acid-resistant acid phosphatase (TRAP) stain, although not specific, is usually positive because of increased levels of an isoenzyme in these cells. The majority of patients present with complications of cytopenia, and 80% will have splenomegaly.

Treatment
Therapy for hairy cell leukemia has undergone a rapid change over the last few years. In fact, the hematologist is now confronted with choosing from more than one highly effective therapy. Historically, initial therapy was splenectomy. This intervention usually improved the cytopenias but only provided transient benefits. Subsequently, interferon-alpha demonstrated significant activity with a response rate of close to 80%. Some patients (about 10%) achieved complete responses. Once interferon was stopped, relapses were common, but frequently patients would respond to another course of treatment with interferon.

Most recently, deoxycoformycin and chlorodeoxy-adenosine (2-CDA) have demonstrated remarkable activity. Deoxycoformycin appears to be active even in those patients who fail interferon, whereas 2-CDA has an extremely high complete response rate and, in fact, may be curative after a single course of therapy.

With such impressive therapy, the indications for initiation of therapy have been reevaluated. In general, up to 20% of patients with hairy cell leukemia have a very indolent form of the disease and may never require therapy. It is important to spare these patients the potential morbidity of treatment. The indications to consider therapy include significant cytopenias, repeated infections, massive or painful splenomegaly or lymphadenopathy, or vasculitis. Close observation for those patients without these indications is warranted.