On flow cytometry, hairy cells show high forward light scatter and slightly higher side light scatter than the normal lymphocytes. They usually fall outside of the lymphocyte region within the monocyte gate (see bottom right panel). HCL expresses bright CD45 and B-cell markers such as CD19, CD20 (bright), CD22, FMC-7, and CD79a. In addition, the neoplastic cells show bright expression of CD11c, CD25, CD103, and CD123. They are usually negative for CD5, CD10, and CD79b. About 10% to 25% of cases with classic features of HCL may show CD10 expression (as seen in this case). It is useful to remember that the vast majority of HCL cases express at least 3 of the following 4 markers : CD11c, CD103, CD25, and CD123. Image courtesy of: Jared Block, MD, Carolinas Pathology, Charlotte, NC; Sciplicity, LLC.