Peyer's patch

Peyer's patches (or aggregated lymphoid nodules, or occasionally PP for brevity) are organized lymphoid nodules, 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, the ileum, in humans; as such, they differentiate the ileum from the duodenum and jejunum. The duodenum can be identified by Brunner's glands. The jejunum has neither Brunner's glands nor Peyer's patches.

Structure

Peyer's patches are observable as elongated thickenings of the intestinal epithelium measuring a few centimeters in length. About 30 are found in humans. 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. The number of Peyer's patches peaks at age 15-25 and then declines during adulthood. 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 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. 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.