Peyer's patches

Peyer's patches are clusters of lymphoid follicles, that is, organized groups of immune cells, tucked into the wall of the small intestine. They sit in the lamina propria and the submucosa, the layers just beneath the inner lining, and they are concentrated mainly in the ileum, the last stretch of the small intestine.

What makes them special is the epithelium that covers them, the layer of cells facing the intestinal lumen, which is the hollow space food travels through. There you find M cells, which unlike ordinary enterocytes lack the usual microvilli and work like customs windows. They grab antigens, which are fragments of bacteria or of anything else that can be recognized, and pass them to the other side in a process called transcytosis, which means crossing the cell from one side to the other.

On the other side there is a welcoming committee. Dendritic cells and macrophages take the sample, and in the germinal centers of the follicles live the B lymphocytes, which mature there. The IgA that is produced, an antibody, is secreted back into the intestinal lumen to deal with whatever is in there.