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Abstract Hairy cell leukemia (HCL) is an indolent lymphoproliferative malignancy characterized by infiltration of the bone marrow, liver, spleen and occasionally lymph nodes with a malignant Bcell with hair-like cytoplasmic projections (Kraut, 2003). These hairy cells co-express CD11c, CD19, CD20, CD22, CD25 and CD103 (Goodman et al, 2003). Recent studies have indicated that it is not uncommon for HCL to display an unusual immunophenotype, including negativity for CD103 or CD25. Recognizing the variability of immunophenotype and correlating with morphologic and clinical features are essential for establishing an accurate diagnosis of HCL (Chen et al, 2006). HCL is usually readily diagnosed by seeing typical hairy cells (HCs) in the blood film. The diagnosis is then confirmed by tartrate-resistant acid phosphatase (TRAP) staining, marker analysis, and bone marrow examination (Allsup and Cawley, 2004). HCL has long been recognized as distinct from other chronic B-cell malignancies, but several questions remain unanswered. What is the HCL cell of origin? Why does HCL lack the hallmarks of most mature B-cell tumours (for example, xv Introduction chromosomal translocations and consistent lymph node involvement) and show unique features like ’hairy’ morphology and bone marrow fibrosis? Gene expression profiling and other studies have recently provided new insights into HCL biology and have the potential to affect clinical practice (Tiacci et al, 2006). The involvement of the reticulo-endothelial system leads to splenomegaly. The common hematological complications of anemia, neutropenia and thrombocytopenia are due not only to the enlarged spleen but probably also to HCs in the bone marrow inducing cytokine mediated suppression of hematopoiesis. Hepatic involvement is also frequent and infections are a major cause of morbidity and mortality in patients with HCL (Kraut, 2003). The development of extremely effective therapy for HCL results in a high incidence of complete remission. However, a significant percentage of patients continue to harbour minimal residual disease that can be revealed with immunohistochemical and flow cytometric studies (Bethel and Sharpe, 2003). |