The intestinal barrier is a component of the innate immune system in the gastrointestinal tract. It is built of three structural components:
- A mucous layer that covers the intestinal epithelium. It is formed of special mucin proteins produced by goblet cells. In addition, antimicrobial proteins and secretory immunoglobulin A (sIgA) are released into the mucus to prevent direct contact between bacteria of the gut microbiome with the epithelium.
- The epithelial monolayer separates the interior of the body from the outside i.e. the intestinal lumen that is exposed to anything ingested. The enterocytes – also responsible for digestion and absorption - are tightly connected and create a physical barrier forcing most molecules to pass through specific channels, transporters and other controlled mechanisms into and through the cells before the molecules enter lymph or blood streams. The space between enterocytes is sealed by specialised proteins, i.e. tight junctions and adhesive junctions.
- The lamina propria. Only the basement membrane separates the lamina propria from the intestinal epithelial monolayer. Many cells of the innate and adaptive immune system are located in the lamina propria.
The intestinal barrier is a semipermeable structure that facilitates selective nutrient absorption while being restrictive against pathogens, most environmental molecules or excess nutrient intake. In addition, it prevents contact between bacteria from the gut microbiome with immune cells, thereby preventing immune responses to beneficial bacteria residing in the gut (Vancamelbeke & Vermeire, 2017).