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.