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). [MeSH]