Esophageal tonsil: a novel gut-associated lymphoid organ.

The esophageal tonsil of the chicken is a novel, significant element of the gut-associated lymphoid tissue (GALT). Its stable location and histological organization fulfills the meaning of the term "tonsil." The six-to-eight-isolated tonsillar units are located at the border of the esophagus and the proventriculus. The number of tonsillar units is identical with that of the esophageal folds. Each tonsillar unit consists of a crypt lined by lymphoepithelium and surrounded by dense lymphoid tissue, which is organized into T- and B-dependent regions, like peripheral lymphoid organs. The excretory ducts of the mucosal glands of the esophagus are frequently involved in the formation of the lymphoepithelium. The esophageal tonsil is anatomically located cranial to the stomach, unlike the other parts of the GALT. Therefore, it is continuously exposed to undigested environmental antigens, allergens, food, and infectious agents. To develop effective oral vaccines, the existence of the esophageal tonsil has to be taken into account.