The association between emperipolesis, defined as the presence of phagocytized cells (mainly lymphocytes, but also plasma cells, neutrophils or erythrocytes) in a histiocyte, and a typical immunohistochemical pattern characterized by positivity for S-100 protein and CD68 antigen and negativity for CD1a antigen, is diagnostic for RDD. Here, CD1A is linked to sinus histiocytosis with massive lymphadenopathy.