Peyer's patches
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'''Peyer's patches''' are secondary lymphoid tissue, named after the 17th-century [[Swiss]] anatomist [[Johann Conrad Peyer]]. They are aggregations of lymphoid tissue that are usually found in the lowest portion of the [[small intestine]] (ileum) in humans; as such, they differentiate the ileum from the [[duodenum]] and [[jejunum]] in that the number of Peyer's patches increase further down the intestine (ie. terminal [[ileum]] contains most Peyer's patches).
==Location and appearance==
Peyer's patches are observable as elongated thickenings of the intestinal epithelium measuring a few centimeters in length. Microscopically, Peyer’s patches appear as oval or round lymphoid follicles (similar to [[lymph nodes]]) located in the lamina propria layer of the mucosa and extending into the submucosa of the [[ileum]].
In adults, B lymphocytes are seen to predominate in the follicles' germinal centers. T lymphocytes are found in the zones between follicles.
==Function==
Because the lumen of the [[gastrointestinal tract]] is exposed to the external environment, much of it is populated with potentially [[pathogen]]ic [[microorganism]]s. Peyer's patches thus establish their importance in the immune surveillance of the intestinal lumen and in facilitating the generation of the immune response within the [[mucosa]].
Pathogenic microorganisms and other [[antigen]]s entering the intestinal tract encounter [[macrophages]], [[dendritic cell]]s, [[B-cells|B-lymphocytes]], and [[T-lymphocytes]] found in Peyer's patches and other [[gut-associated lymphoid tissue]] (GALT).
Peyer's patches are covered by a special epithelium that contains specialized cells called [[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). [[B-cell]]s 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.
==Pathology==
Although important in the immune response, excessive growth of lymphoid tissue in Peyer’s patches is pathologic, as [[Organ hypertrophy|hypertrophy]] of Peyer’s patches has been closely associated with idiopathic [[intussusception (medical disorder)|intussusception]].
The hypertrophy of Peyer's patches has also been associated with susceptibility to transmissible spongiform encephalopathies (commonly known as prion diseases).
{{lymphatic_system}}
{{Digestive tract}}
[[Category:Lymphatic system]]
[[de:Peyer-Plaques]]
[[es:Placas de Peyer]]
[[nl:Peyerse plaat]]
[[ja:パイエル板]]
[[pl:Kępka Peyera]]
[[sv:Peyers plack]]
[[uk:Пеєрові бляшки]]