Unusual expression of S‐100 protein in histiocytic neoplasms

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Abstract
Twenty‐nine cases of histiocytic neoplasms, some resembling juvenile xanthogranuloma (JXG) and others resembling reticulohistiocytoma (RH), were evaluated. Immunohistochemical stains were performed. In this series, seven cases were identified that expressed S‐100 protein positive cells. The S‐100 positive cells were predominantly large mononuelear and multinucleated histiocytes with eosinophilic cytoplasm, but also in some cases xanthomatous cells and Touton giant cells. These cells also expressed a positive reaction for vimentin, KP‐1, and Factor XIIIa. There was no reactivity observed for monoclonal antibody 010(CD1a). A positive reaction for S‐100 protein is conventionally accepted as a useful differentiating feature between histioeytosis X and non‐X histiocytosis such as JXG and RH. The conflicting results of the immunohistochemical stains in the lesions we studied could be potential pitfalls in diagnosing histiocytic neoplasms.