The overlying 'dome' epithelium contains large numbers of intraepithelial lymphocytes. Some of the epithelial cells have complex microfolds in their surfaces. They are known as M cells and are believed to be important in the transfer of antigen from the gut lumen to Peyer's Patches. Peyer's Patches facilitate the generation of an immune response within the mucosa. B cell precursors and memory cells are stimulated by antigen in Peyer's Patches.
Cells pass to the mesenteric lymph nodes where the immune response is amplified.
Activated lymphocytes pass into the blood stream via the thoracic duct. These cells then home in the gut and carry out their final effector functions. HEVs are not present in Peyer's Patches and the mechanism by which cells home in on mucosal sites is unknown. Cell surface molecules known as addressins may have a role. The complex patterns of recircultion depend on the state of activation of the lymphocytes, the adhesion molecules expressed by endothelial cells and the presence of chemotactic molecules which selectively attract particular populations of lymphocytes or macrophages.