Fig. 1.

Components of intestinal barrier function. The gut lumen is where the degradation of bacteria and antigens by bile, gastric acid and pancreatic juice occurs. Commensal bacteria inhibit the colonization of pathogens through production of defense antimicrobial substances. The unstirred water layer, glycocalyx and thick/thin mucus layer works to prevent bacterial adhesion by immunoglobulin A (IgA) secretion. Within the epithelium, tight junctions (TJs) connect the epithelial cells to transport luminal contents. TJ-associated proteins (occludin, zonula occludens-1, claudin-1/4, junctional adhesion molecules) are vital to barrier integrity. Myosin light chain activation regulates the contraction and tension of actin which allows for opening of the paracellular pathways. The lamina propria is home to innate and acquired immune cells which secrete immunoglobulins and cytokines for use within the endocrine and enteric nervous system. Disruption of the intestinal barrier with permeation of noxious molecules to the lamina propria, induces mucosal immune system activation and inflammation. SCFA short chain fatty acids, IgA immunoglobulin A, IL interleukin, ILCs innate lymphoid cells, Th t helper cells, Treg T regulatory cells, ZO-1 zonula occludens-1, MLCK myosin light-chain kinase (From: Camilleri M, Lyle BJ, Madsen KL, Sonnenburg J, Verbeke K & Wu GD. Role for diet in normal gut barrier function: developing guidance within the framework of food-labeling regulations. Am J Physiol Gastrointest Liver Physiol. 2019 Jul 1;317(1):G17-G39; used with permission of Mayo Foundation for Medical Education and Research, all rights reserved)