(a), (b) [6]. Histiocytic cells with emperipolesis are immunoreactive for S-100 protein and negative for EMA. (a) Photomicrograph (H and E, original magnification, ×400) shows large histiocytes that display prominent intracytoplasmic lymphocytes (arrow). (b) Photomicrograph (S100 immunostaining, original magnification, ×400) shows diffuse, strong cytoplasmic positivity within histiocytes (arrow). Emperipolesis, that is, engulfment by Rosai-Dorfman disease histiocytes of lymphocytes and other reactive inflammatory cells is evident in this image (a).
