The expression of indoleamine 2,3-dioxygenase (IDO) in biopsies from the nasal cavity. (A) Nasal mucosa taken from a patient with allergic rhinitis off-season (during winter). No epithelial or leukocyte expression of IDO. (B) Specimen from another patient with allergic rhinitis, taken off-season, has epithelial IDO+ and leukocyte IDO-. (C) The same allergic rhinitis patient as in frame A, did not display any IDO expression in the specimen taken in-season (during spring). (D) Specimen from a healthy patient, taken off-season, shows epithelial IDO- and leukocyte IDO+. Original magnification 200× (A, B, D) and 400× (C).