Atypical histiocytic granuloma. Light microscopic, ultrastructural, and histochemical findings in an unusual pseudomalignant reactive lesion of the oral cavity.

Four cases of an ulcero-proliferative lesion of putative histiocytic origin, arising in the oral cavity, are described. This lesion, which has not been defined previously, is easily confused histologically with a malignant process of lymphoid origin, thereby representing a pseudolymphomatous proliferation. Light microscopic, ultrastructural, and histochemical characteristics including immunoglobulin negativity, positive esterase reactivity, and positive peanut lectin binding support a histiocytic origin. These lesions are not associated with systemic disease or dissemination and heal spontaneously despite their worrisome microscopic appearance.