The defences in the gut
A major function of the gut is to absorb water and nutrients, which is why it has such a large surface area and is covered by just a single layer of epithelium. But the gut is also the main entry point for foreign substances into the body, including potentially harmful microbes, which is why many forms of defence are located in the gut. For example, approximately 70% of all immunoglobulin A ( IgA; a major antibody of mucosal surfaces) is produced in the gut and immune responses initiated here can affect the whole body. The overall protective function of the gut is referred to as the intestinal barrier function.
The lines of defence in the gut comprise:
- Chemical secretions, e.g. gastric and pancreatic juices, deconjugated bile acids, antimicrobial peptides (e.g. defensins), enzymes
- The commensal microbiota, (i) competing with endogenous microbes for nutrients and adhesion sites (colonisation resistance; competitive exclusion); (ii) educating and supporting the immune response; (iii) reducing or neutralising harmful dietary components and/or microbial metabolites
- The mucus layer, helping to prevent direct injury and bacteria translocation
- The intestinal epithelium, acting as a barrier by maintaining the integrity of tight junctions
- The gut-associated lymphoid tissue, comprising Peyers patches, lymphocytes, dendritic cells, cytokines, etc.