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  • A B-lymphocyte malignancy principally involving 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 1000 cases per year occur in the United States (~2% of all leukemias).

  • The male-to-female ratio is approximately 4:1. This large difference is unexplained.

  • 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 of classical hairy cell leukemia (HCL-c).

  • BRAF V600E is present in the hemopoietic stem cells of individuals with HCL-c.

  • This mutation is not found in cases of variant hairy cell leukemia (HCL-v).

  • Hairy cells are B cells in a late (pre–plasma cell) stage of development.

  • Neoplastic B cells have clonal immunoglobulin gene rearrangements.

  • Neoplastic B cells express pan–B-cell markers (eg, CD19, CD20strong, CD22, and CD123) 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).

  • HCL-c cells do not express CD5, CD10, CD27, or CD43.

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


  • Fatigue and weakness, with or without weight loss, are the most common presenting symptom that occur (50%).

  • Abdominal fullness/discomfort is caused by massive splenomegaly.

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

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

  • Painful bony lesions rarely occur (3%).

  • Splenomegaly exists in 90% of patients, but the incidence is falling due to earlier diagnosis.

  • Infections with common bacteria, viruses, fungi, Mycobacterium kansasii, Pneumocystis jirovecii, 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.

  • Paraneoplastic neurological syndromes occur rarely.


  • 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 a hallmark of the disease.

  • Thrombocytopenia was present at diagnosis in about 75% of patients in the cohorts reported several decades ago, but subsequent earlier diagnosis has reduced this frequency. The extent of thrombocytopenia is very variable but is rarely less than 20 × 109/L.

  • Moderate to ...

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