Histologic features of the rectal mucosa. (A) Rectal biopsy of patient P26 showing very mild lamina propria infiltration by lymphocytes and macrophages. (B) No MHC class II expression on inflammatory and epithelial cells. (C) Rectal biopsy of patient P14 revealing moderate lamina propria infiltration by lymphocytes, eosinophils, macrophages, and plasma cells. (D) Absence of MHC class II expression on inflammatory and epithelial cells. Only eosinophils are stained because of endogenous peroxidase activity. (E) Normal histologic appearance of a control rectal biopsy. (F) Normal expression of MHC class II on lamina propria cells in a rectal biopsy.