Group of disorders which feature accumulations of active histiocytes and lymphocytes, but where the histiocytes are not langerhans cells. The group includes HEMOPHAGOCYTIC LYMPHOHISTIOCYTOSIS; SINUS histiocytosis; xanthogranuloma; reticulohistiocytoma; JUVENILE XANTHOGRANULOMA; xanthoma disseminatum; as well as the lipid storage diseases (sea-blue histiocyte syndrome; and niemann-pick diseases).