The intestinal mucus as protective immunity
The mucus is not only a physical barrier: also influences the immunogenic capacity by increasing the intestinal immune tolerance, namely the lack of response of lymphocytes to the antigen. The mechanisms by which the intestinal mucous support commensal bacteria and food antigens without developing inflammation remain elusive and although it is traditionally seen as a barrier between host and non-specific environment, the mucus also regulates intestinal homeostasis. The digestive system is subject to a delicate balance, because the immune system has to protect it from harmful external bacteria while respecting the resident intestinal flora. If the immune system mistakenly attacks the friendly bacteria develops intestinal inflammation that could contribute to the genesis of chronic diseases such as Crohn’s disease and ulcerative colitis, while the mucus helps to stop this immune response by forming a physical barrier between epithelium and intestinal contents.
In the large intestine mucus prevents inflammation, generating two layers of mucous different from each other: one external inhabited by gut microbiota, and one inside, adherent to the epithelial cells and impermeable to bacteria.
Actually is little known the functions of small intestine and researchers have suggested a possible active role in preventing the immune response to foreign antigens. By goblet cells in the small intestine has highlighted the role of a protein, MUC2, capable of retaining water molecules, making the mucus thick and sticky.
Laboratory experiments with mice and human cells have shown that the mucus is not just a passive viewer, but is also able to suppress the immune response to compounds that would otherwise lead to inflammation of the intestine.
The barrier mucus lining of the small intestine is porous and allows the bacteria to pass through its surface, in contrast with the mucus of the large intestine that forms only a dense barrier to protect the surface.
So, the bacteria that arrive in the small intestine are coated by MUC2 and phagocytosed by cells of the immune system without that these are activated producing inflammatory mediators as would happen with bacteria not coated with mucus: the intestinal mucin not only acts as a physical barrier, but has a tolerant genic effect to the foreign substances that come in the small intestine, helping to prevent the inflammatory responses.
According to researchers, the long-term goal is that these findings will help to better understand the role of mucus, helping to develop new ways to prevent and treat intestinal infections, food allergies and inflammatory bowel diseases such as ulcerative colitis and Crohn’s disease .