[Lymphoid follicles of the human large bowel mucosa: structure and function]

Little is known about morphology and function of human solitary lymphoid follicles (LF), a component of the gut-associated lymphoid tissue. We have studied 156 LF in situ from 10 colorectal resection specimens by use of serial frozen sections and immunohistochemistry (APAAP technique) and found 8 to 10 LF per cm2, i.e. 11,000 to 14,000 LF in the whole organ. Two types were identified: (1) One type, located in the colon, displayed a pit-like follicle-associated epithelium (FAE). A large part of this LF extended into the submucosa. (2) The other type was noticed mainly in the tunica mucosa of the rectum and had a flat FAE. All LF exhibited a rather constant structure. The main elements were FAE, dome region, B-lymphocyte region with surrounding T-lymphocyte area including high endothelial venules, and a lymph sinus that delimited the LF against the submucosa. One out of five LF contained a germinal center. Marked differences were found between the immunological effector cell population of the LF and that of the lamina propria. These observations denote (1) a striking compartmentation of these cells in the gut mucosa; (2) a marked similarity of the immunomorphology of LF to that of Peyer patches; (3) a functional analogy: The LF belong to a physically discrete compartment that constitutes the afferent limb of the local immune system, as the Peyer patch does. The lymphomonocytic elements of the lamina propria belong to the efferent limb. Results of in vitro studies, generally based on a mixture of cells from both compartments, may thus not reflect the conditions found in vivo.