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  • This B-lymphocyte malignancy principally involves the marrow and spleen.

  • Blood cytopenias and marrow reticulin fibrosis are frequent features.

  • Irregular cytoplasmic projections on neoplastic B lymphocytes (which gives the disease its name) are most striking when examined as a wet preparation by phase microscopy.


  • It is estimated that about 700 cases per year occur in the United States (approximately 2% of all leukemias).

  • The male-to-female ratio is approximately 4:1.

  • The median age at presentation is approximately 55 years.

  • There is a bimodal peak incidence by age with a mode at approximately age 30 and at age 55.

  • More than 90% of patients are of European descent.

  • Disease is rare in persons of African or Asian descent.


  • No exogenous causes have been established.

  • A mutation in BRAF (ie, BRAF V600E) is found in virtually all cases. This mutation is not found in cases of variant hairy cell leukemia.

  • Hairy cells are B cells in a late (preplasma cell) stage of development.

  • Neoplastic B cells have clonal immunoglobulin gene rearrangements.

  • Neoplastic B cells express pan-B–cell markers (eg, CD19, CD20, and CD22) and the plasma cell marker prostate cancer antigen-1.

  • Neoplastic B cells express additional surface antigens that are uncommon on B lymphocytes (eg, CD11c, CD25, and CD103).

  • Neoplastic B cells secrete cytokines that may impair normal hematopoiesis (eg, tumor necrosis factor-α).


  • Abdominal fullness/discomfort is caused by massive splenomegaly (25%).

  • Fatigue, weakness, and weight loss may occur (25%).

  • Bleeding or infection may be present (25%).

  • Some patients are found incidentally to have abnormal blood count and/or splenomegaly (25%).

  • Painful bony lesions rarely occur (3%).

  • Splenomegaly exists in 90% of patients (median splenic weight approximately 1300 g).

  • Infections with common bacteria, viruses, fungi, Mycobacterium kansasii, Pneumocystis jiroveci, aspergillus, histoplasma, cryptococcus, Toxoplasma gondii or other opportunistic organisms, once common, are less frequent because of more effective initial therapy.

  • Unusual findings include cutaneous vasculitis, leukoclastic angiitis, erythema nodosum, polyarthritis, and Raynaud phenomenon.


  • Anemia is present in three-fourths of patients.

  • Eighty percent of patients have absolute neutropenia and monocytopenia.

  • Severe neutropenia (< 0.5 × 109/L) is found in 30% of patients.

  • Severe monocytopenia is hallmark of the disease.

  • Thrombocytopenia occurs in about 75% of patients.

  • Moderate to severe pancytopenia is found in approximately 70% of patients.

  • Careful examination of the blood by light microscopy identifies hairy cells in 80% of patients (Figure 56–1).

  • Liver function test abnormalities occur in 19% of cases, azotemia in 27%, and hyperglobulinemia in 18%, which may be monoclonal.

  • Occasionally, leukocytosis is present as a result of circulating hairy cells. Extreme leukocytosis (> 100 × 109/L) can occur very infrequently, most often seen in the “hairy cell leukemia variant” (see below).

  • Hairy cells comprise less than 20% of lymphocytes in ...

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