Barrier function of the gastrointestinal tract

The gastrointestinal epithelium is the layer of cells that marks the boundary between self and non-self. It serves the crucial roles of nutrient and water uptake while simultaneously blocking entrance of harmful substances and pathogens into the body. This barrier function comprises three layers: (1) the extrinsic layer consisting of commensal bacteria and secretions, (2) the intrinsic layer forming the physical barrier with cells and cell junctions such as tight junctions, and finally (3) the immunological layer consisting of the local immune system of the gastrointestinal wall. This richly illustrated review describes the anatomy and function of these layers of barrier. We also describe how various factors such as stress, diet, inflammation, and disease can affect the gastrointestinal barrier function.