Numerous lymphoid organs found in the wall of the small intestine, which have a crucial role in protecting against infection. Each consists of a dome-shaped follicle containing chiefly B cells surrounded by smaller numbers of T cells. Within the surface of the follicle exposed to the gut lumen are specialized epithelial cells called M cells (multifenestrated cells). These have a microfolded luminal surface, and take up particles from the gut by endocytosis. The particles are then transferred through the M cells to the underlying lymphocytes for antigen processing. Within the follicle, dendritic cells present antigen to naive lymphocytes, which become activated and leave Peyer’s patches, returning to the blood via mesenteric lymph nodes and the thoracic duct. These then circulate in the blood and take up station as effector cells at various mucosal sites in the body, including the intestinal epithelium, primed in readiness for a subsequent encounter with the antigen. The organs were named after Swiss anatomist Johann K. Peyer (1653–1712).