much of it is populated with potentially pathogenic microorganisms. Peyer's patches thus establish their importance in the immune surveillance of the intestinal lumen and in facilitating production of the immune response within the mucosa. Pathogenic microorganisms and other antigens entering the intestinal tract encounter macrophages, dendritic cells, B-lymphocytes, and T-lymphocytes found in Peyer's patches and other sites of gut-associated lymphoid tissue (GALT). Peyer's patches thus act for the gastrointestinal system much as the tonsils act for the respiratory system, trapping foreign particles, surveilling them, and destroying them. Peyer's patches are covered by a special follicle-associated epithelium that contains specialized cells called microfold cells (M cells) which sample antigen directly from the lumen and deliver it to antigen-presenting cells (located in a unique pocket-like structure on their basolateral side). Dendritic cells and macrophages can also directly sample the lumen by extending dendrites through transcellular M cell-specific pores. At the same time the paracellular pathway of follicle-associated epithelium is closed tightly to prevent penetration of antigens and continuous contact with immune cells. T cells, B-cells and memory cells are stimulated upon encountering antigen in Peyer's patches. These cells then pass to the mesenteric lymph nodes where the immune response is amplified. Activated lymphocytes pass into the blood stream via the thoracic duct and travel to the gut where they carry out their final effector functions. The maturation of B-lymphocytes takes place in the Peyer's patch. Clinical significance Although important in the immune response, excessive growth of lymphoid tissue in Peyer's patches is pathologic, as hypertrophy of Peyer's patches has been closely associated with idiopathic intussusception. Having too many or larger than normal Peyer's patches is associated with an increased risk of prion diseases. Salmonella typhi and poliovirus also target this section of the intestine. See also Influenza Lung cancer References External links Lymphatic system Lymphatic tissue Small intestine