Unlike the term “dermatitis” as used by dermatology practitioners and dermatopathologists to refer to superficial skin pathology involving the epidermis and papillary dermis and that may be spongiotic, interface, or psoriasiform; the use of the term dermatitis in “granulomatous dermatitis” is loose. It simply refers to a process in the skin in which the infiltrate is granulomatous, that is, primarily histiocytic the term is used by general pathologists more than dermatopathologists.

A few undifferentiated monocytes may be present in a primarily lymphocytic infiltrate . These are hard to differentiate from lymphocytes under light microscopy. When a monocyte becomes a histiocyte it acquires phagocytic function and morphological characteristics that makes it easily detectable by routine histology. It becomes larger, its cytoplasm abundant, and often acquires an Idaho potato appearance, or it becomes an “epithelioid cell,” meaning, it becomes rounded and closely packed with other similar cells resembling epithelial cells. Organisms such as leprosy organisms or material such as lipid, melanin, or iron may be visible in its cytoplasm, so it may be called a lepra cell, xanthoma cell, melanophage, or siderophage .