As reviewed in Chapter 28, hairy cell leukemia (HCL) is a B-cell malignancy comprising 2% of all leukemias,12 and is highly responsive, but not curable, with known therapy. Patients present with pancytopenia and splenomegaly, and have malignant cells in the blood, bone marrow, and spleen with eccentric kidney-shaped spongiform nuclei and hairlike cytoplasmic projections. The diagnosis was classically made by tartrate-resistant acid phosphatase (TRAP) staining.34 More recently, the diagnosis is most accurately made by fluorescent-activated cell sorting (FACS) analysis (or flow cytometry), which demonstrates HCL cells strongly positive for B-cell antigens CD19, CD20, and CD22, and often other antigens including CD103 (B-ly7) CD11c, CD25, and CD123.5-7 The differential diagnosis, which is important to consider in relapsed or refractory patients, includes splenic lymphoma with villous lymphocytes (SLVL), which may be distinguished from HCL by polar distribution of villous cytoplasmic projections, lack of pancytopenia, and CD25, CD103, and/or CD123 negativity.7-9 HCL variant (HCLv) is a disorder comprising about 20% of HCL in which the malignant cells may have bilobed nuclei, prominent nucleoli, and may be clumped in the marrow. The HCLv cells may be CD25, CD103, CD11c, and/or CD123 negative, and patients are primarily refractory to purine analogs and other thera-pies.6'7'10-12

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