Histologic and immunohistochemical features of NK-cell enteropathy. Representative cases are shown involving stomach (A-D), duodenum (E), and colon (F). (A) The atypical cells infiltrate the mucosa, separating and displacing the glands. (B) Epitheliotropism is largely absent, but the cells show cytologic atypia, with irregular nuclei and eosinophilic, granular cytoplasm. (C) NK cells are strongly positive for CD56 and diffusely infiltrate the mucosa, with rare cells infiltrating the glandular epithelium. The infiltrate does not extend to the submucosa. (D) The cells express cytoplasmic CD3 (D), and immunohistochemical stains highlight the cytologic atypia. (E) CD56 identifies atypical NK cells diffusely infiltrating and expanding duodenal villi. (F) Atypical cells surround colonic glands with focal areas of superficial hemorrhage. (C-E) Immunohistochemistry with avidin-biotin-peroxidase complex immunoperoxidase technique and hematoxylin counterstain. Photomicrographic images were acquired with a Nikon Eclipse 50i microscope equipped with an Olympus DP71 camera and software. Final image preparation was performed with Adobe Photoshop CS3 extended Version 10.0.1. Original magnifications: panel A, 4×/0.2 NA; panel B, 40×/0.95 NA; panel C, 10×/0.45 NA; panel D, 100×/1.40 NA; panel E, 10×/0.45 NA; panel F, 20×/0.75 NA.