Fig. 1.

Intestinal barrier structure. The mechanical barrier includes intact epithelial cells, a lipid bilayer with tight brush borders of epithelial cells, and cell junctions at the lateral borders of the cells. The chemical barrier consists of digestive juices secreted by the gastrointestinal tract, digestive enzymes, and antibacterial substances produced by the normal intestinal microbiota. The immune barrier is primarily composed of gut-associated lymphoid tissue and secretory immunoglobulins. The biological barrier represents an interdependent and interacting microecosystem composed of the resident intestinal microbiota