Peyer's patch

Peyer's patches (or aggregated lymphoid nodules, or occasionally PP for brevity) are organized lymphoid follicles, named after the 17th-century Swiss anatomist Johann Conrad Peyer. They are an important part of gut associated lymphoid tissue usually found in humans in the lowest portion of the small intestine, mainly in the distal jejunum and the ileum, but also could be detected in the duodenum.

Appearance
Peyer's patches looks like a bar with many dummy stuff for the cell to hide in, waiting for the annoying bacteria coming in.