The gut barrier is a key immune barrier in the human body, consisting of a layer of intestinal epithelial cells and a mucus layer. The human intestinal epithelium covers a surface area of up to 400 square meters. Intestinal epithelial cells are linked together by tight junction proteins. They keep substances in the gut lumen, including pathogens (viruses and bacteria) and food debris, from entering the bloodstream. If the seal between the epithelial cells of the intestinal tract is reduced, like a door that is not closed tightly and has a gap, the bacteria and toxins in the gut lumen will "leak." This can trigger various adverse reactions, such as inflammation and some chronic diseases.