Fig. 1

The three interactive layers of intestinal barrier. The luminal layer consists of the intestinal commensal organisms that spatially inhibit colonization of the intestine with pathogenic microbial species. The middle layer consists of mucus that prevents the contact of the mucosal epithelium with the intestinal micro-organisms. This mucus layer not only provides a physical barrier, it also contains antimicrobial proteins and IgA, both of which contribute further to protect the mucosal epithelium. The mucosal epithelium provides a selectively permeable barrier to the luminal contents