Peyers

Peyer's patches are organized lymphoid follicles that form a major component of the gut-associated lymphoid tissue (GALT) in the ileum, the final portion of the small intestine. They appear as discrete aggregates of lymphoid tissue embedded in the submucosa and are most abundant in the distal ileum. The patches are covered by a specialized epithelium called follicle-associated epithelium, which includes M cells that sample luminal antigens and transport them to underlying immune cells.

Histologically, each patch contains densely packed B-cell follicles with germinal centers, surrounded by T cells in the interfollicular area. Dendritic cells and other antigen-presenting cells reside within the region to present antigens to lymphocytes. Multiple patches drain via lymphatics to regional mesenteric lymph nodes. They may enlarge during infection or inflammation.

Functionally, Peyer's patches monitor intestinal contents for pathogens and commensal organisms, initiating mucosal immune responses. They contribute to the generation of IgA-producing plasma cells and support class switching to immunoglobulin A, which helps maintain intestinal immunity and barrier function. They also participate in oral tolerance, helping the immune system distinguish harmless dietary antigens from pathogens.

History and clinical notes: the structure is named after Johann Conrad Peyer, an 17th–18th century Swiss anatomist. In health, Peyer's patches support normal mucosal immunity; in disease, they can exhibit lymphoid hyperplasia and may be involved in certain inflammatory conditions of the gut, such as Crohn’s disease. In humans and many animals, they play a conserved role in gut immune defense.