M cells, which transport antigens across the intestinal epithelium, are most abundant in Peyer's patches located in which segment of the small intestine?

• A. Ileum
• B. Duodenum
• C. Jejunum
• D. Cecum

Answer: (A)

M cells are specialized epithelial cells that sample antigens from the gut lumen and deliver them to underlying immune cells in Peyer’s patches, forming a direct link between the intestinal surface and mucosal immunity. The density of M cells tracks with the density of Peyer’s patches, which are most numerous in the ileum, especially the distal region. This location supports efficient antigen sampling as most gut antigens encountered pass through this area, enabling dendritic cells and lymphocytes beneath the epithelium to mount appropriate immune responses, including IgA production. While Peyer’s patches exist elsewhere in the small intestine, they are far less abundant there, and the cecum is in the large intestine, not the small intestine.