Editing Peyer's patch (section)

== Structure ==
Peyer's patches are observable as elongated thickenings of the [[intestinal mucosa]] measuring a few centimeters in length. About 100 are found in humans. Microscopically, Peyer's patches appear as oval or round lymphoid follicles (similar to [[lymph nodes]]) located in the [[mucosa]] layer of the [[ileum]] and extend into the [[submucosa]] layer.
The number of Peyer's patches peaks at age 15–25 and then declines during adulthood.<ref name="Cornes J S 1965"/>
In the distal ileum, they are numerous and they form a lymphoid ring. At least 46% of Peyer's patches are concentrated in the distal 25&nbsp;cm of ileum in humans. It is important to note that there are large variations in size, shape, and distribution of Peyer's patches from one individual to another one.<ref>{{cite journal | vauthors = Van Kruiningen HJ, West AB, Freda BJ, Holmes KA | title = Distribution of Peyer's patches in the distal ileum | journal = Inflammatory Bowel Diseases | volume = 8 | issue = 3 | pages = 180–5 | date = May 2002 | pmid = 11979138 | doi = 10.1097/00054725-200205000-00004 | s2cid = 22514793 | doi-access = free }}</ref>
In adults, B lymphocytes are seen to dominate the follicles' germinal centers.  [[T lymphocytes]] are found in the zones between follicles. Among the mononuclear cells, CD4+/CD25+ (10%) cells and CD8+/CD25+ (5%) cells are more abundant in Peyer's patches than in the peripheral blood.<ref name="pmid21188221">{{cite journal | vauthors = Jung C, Hugot JP, Barreau F | title = Peyer's Patches: The Immune Sensors of the Intestine | journal = International Journal of Inflammation | volume = 2010 | pages = 823710 | date = September 2010 | pmid = 21188221 | pmc = 3004000 | doi = 10.4061/2010/823710 | doi-access = free }}</ref>

Peyer's patches are characterized by the follicle-associated epithelium (FAE), which covers all lymphoid follicles.<ref name="ReferenceA">{{cite journal | vauthors = Owen RL, Jones AL | title = Epithelial cell specialization within human Peyer's patches: an ultrastructural study of intestinal lymphoid follicles | journal = Gastroenterology | volume = 66 | issue = 2 | pages = 189–203 | date = February 1974 | pmid = 4810912 | doi=10.1016/s0016-5085(74)80102-2| doi-access = free }}</ref> FAE differs from typical small [[intestinal villus]] epithelium: it has fewer [[goblet cells]]<ref>{{cite journal | vauthors = Onori P, Franchitto A, Sferra R, Vetuschi A, Gaudio E | title = Peyer's patches epithelium in the rat: a morphological, immunohistochemical, and morphometrical study | journal = Digestive Diseases and Sciences | volume = 46 | issue = 5 | pages = 1095–104 | date = May 2001 | pmid = 11341655 | doi=10.1023/a:1010778532240| s2cid = 34204173 }}</ref> therefore mucus layer is thinner,<ref>{{cite journal | vauthors = Ermund A, Gustafsson JK, Hansson GC, Keita AV | title = Mucus properties and goblet cell quantification in mouse, rat and human ileal Peyer's patches | journal = PLOS ONE | volume = 8 | issue = 12 | pages = e83688 | year = 2013 | pmid = 24358305 | doi = 10.1371/journal.pone.0083688 | pmc = 3865249 | bibcode = 2013PLoSO...883688E | doi-access = free }}</ref> and it is also characterized by the presence of specialized M cells or [[microfold cells]], which provide uptake and transport of antigens from lumen.<ref name="ReferenceA"/> Moreover, [[basal lamina]] of follicle-associated epithelium is more porous compared to intestinal villus.<ref>{{cite journal | vauthors = Takeuchi T, Gonda T | title = Distribution of the pores of epithelial basement membrane in the rat small intestine | journal = The Journal of Veterinary Medical Science | volume = 66 | issue = 6 | pages = 695–700 | date = June 2004 | pmid = 15240945 | doi=10.1292/jvms.66.695| doi-access = free }}</ref> Finally, follicle-associated epithelium is less permeable for ions and macromolecules, basically due to higher expression of [[tight junction]] proteins.<ref>{{cite journal | vauthors = Markov AG, Falchuk EL, Kruglova NM, Radloff J, Amasheh S | title = Claudin expression in follicle-associated epithelium of rat Peyer's patches defines a major restriction of the paracellular pathway | journal = Acta Physiologica | volume = 216 | issue = 1 | pages = 112–9 | date = January 2016 | pmid = 26228735 | doi = 10.1111/apha.12559 | hdl = 11701/6438 | s2cid = 13389571 | hdl-access = free }}</ref>