A case of cutaneous mastocytosis in a child with prominent Langerhans cell infiltration.

We recently encountered a 2-year-old boy with slightly infiltrative brown papules on the face, trunk, and extremities. Stroking of one of the papules produced an urticarial wheal (positive Darier's sign). Histopathologic tests revealed a dense infiltration of mast cells containing numerous granules and showing metachromasia under Toluidine blue staining. Immunohistochemical tests revealed that these cells were positive for CD68 and for c-kit. In addition, dermal dendritic cells that were positive for S100 and CD1a immunostaining were intermingled with the mast cells. We confirmed through electron microscopy that the dermal dendritic cells that were observed adjacent to the infiltration of mast cells had Birbeck granules in their cytoplasm, namely Langerhans cells. However, because of the greater numbers of mast cells than Langerhans cells, and because of the absence of both monomorphic LC proliferation and systemic symptoms of Langerhans cell histiocytosis, the present case favors a diagnosis of cutaneous mastocytosis in a child with Langerhans cell infiltration.