Mixed hyperplastic and neoplastic polyp of the colon. An immunohistological study.

A small colonic polyp which was composed of equal parts of hyperplastic and adenomatous tissue sharing a common basement membrane, displayed a paradoxical distribution of immunohistological markers: In contrast with the neoplastic component, the hyperplastic tissue lacked signs of functional maturity (IgA, secretory component) and displayed markers associated with carcinoma (carcinoembryonic antigen, peanut-agglutinin binding).