Did you know that the inside of your digestive tract is in direct contact with the outside environment? This may seem counterintuitive, but it is a fact: it runs through your body lengthwise. This makes it one of our body’s key areas of exchange with the outside. Like the skin, it is an area at risk of invasion where pathogens, such as bacteria and viruses, can be found. The digestive tract therefore has an important barrier role to play, hence the name “intestinal barrier“, to ensure the protection of our body, in addition to its role of digestion of food and absorption of nutrients. How is the intestinal barrier organized? How does it distinguish between nutrients, beneficial microorganisms, and pathogenic microorganisms? Let’s find out.
Intestinal barrier: 3 layers regulating its functions
Mucus is the superficial layer of the intestinal barrier, the one that is in direct contact with the outside. In other words, it is in contact with the food being digested, the intestinal microbiota, but also any dangerous microorganisms that we may ingest. The cells of our digestive tract are lined with mucus, forming an initial physical barrier of protection against external aggressions. But that’s not all, mucus also maintains a close relationship with the intestinal microbiota. Both contribute to the strengthening of this barrier to exclude pathogens. A formidable first layer of protection.
The second layer of the intestinal barrier is the so-called “intestinal epithelium“. It is made up of a single layer of epithelial cells, very diverse cells, including those that produce mucus. They are firmly attached to each other by “intercellular junctions“. These are sets of proteins that hold the lining of the digestive tract together, preventing unwanted components from passing between the cells and entering the body.
Remember, the digestive tract must also absorb water and nutrients. So how do they get through this solid layer of cells? It turns out that epithelial cells are equipped with “transporters“. These protein complexes act as a gateway to one side of the cell and a gateway to the opposite side, allowing molecules that are essential to the body’s functioning to pass through the epithelial cells into the bloodstream.
Underneath the intestinal epithelium is the third line of defense: the immune cells. In fact, 70% of all our immune cells are located in the intestine, where they play a preventive role. Some of them produce antibodies that are continuously released into the mucus to reinforce its barrier properties. If, despite this, a foreign body manages to penetrate this level of the intestinal barrier, the alarm is sounded. The epithelial cells produce substances to signal to the immune cells that an invasion is taking place, so that the latter can eliminate the danger and thus avoid a potential infection.
To avoid unnecessary alarms, the immune system of the intestine acquires a certain tolerance to the presence of food and bacteria of our own microbiota. This tolerance develops from a very young age. During the first months of life, these immune cells learn to distinguish between elements that could constitute a danger and others (e.g., food, “friendly” bacteria of the microbiota etc.). This learning process helps to avoid the frequent triggering of unjustified immune reactions. Autoimmune diseases, such as, food allergies or IBD, are examples of pathologies where the immune cells of the intestine overreact unnecessarily, which can have serious consequences for health.
Thanks to these different components, the intestinal barrier is a very good means of defense against the invasion of pathogens. However, in some cases, the intestinal barrier can deteriorate and lose its sealing, which is called intestinal hyperpermeability. What are the causes and, above all, how can we prevent it? Find out more in our dedicated article.
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 Vighi C, et al. Allergy and the gastrointestinal system. Clin Exp Immunol 2008