Hairy cell leukemia (HCL) is an entity that is not immunophenotypically difficult to distinguish from CLL, but it is included in the differential diagnosis of CLL in this chapter because of its historic inclusion in the differential diagnosis of chronic lymphoproliferative disorders. Immunophenotypically, HCL demonstrates very little overlap with CLL. HCL exhibits bright expression of the pan-B-cell markers CD19, CD20, and CD22, whereas CD5 and CD23 are negative. Classical features of HCL also include positivity for CD11c, CD25, and CD103. The pattern of CD11c/CD20 positively is highly characteristic of this disease (38).
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