**2. Conclusions**

The gastrointestinal system is, together with the skin and the respiratory system, the habitat most exposed to the external environment, microorganisms and compounds derived from digestion. This condition requires a complex defense system capable of separating the intestinal contents from the host tissues, regulating the absorption of nutrients and allowing interaction between the resident microbial flora and the mucosal immune system, inhibiting the translocation of pathogens into the underlying tissues. All these functions are performed by the intestinal barrier, a functional unit, organized as a multi-layered system: The barrier is more superficially composed of a physical surface barrier, which prevents bacterial adhesion and regulates the paracellular diffusion towards the underlying host tissues. More in depth, we find a deeper functional barrier, which is able to discriminate between commensal and pathogenic microorganisms, organizing the immunological tolerance towards the commensal bacteria and the immune response towards the

pathogens. The fundamental elements on which the integrity and functionality of the "intestinal barrier" depends are therefore the IECs and the intercellular junctions. Glutamine plays a fundamental role in the metabolism of IECs. A condition of eubiosis involves the correct synthesis/absorption of glutamine and glutathione by the IECs. Furthermore, the presence of "healthy" bacterial species producing NEFAs in the correct proportion, with an excess of butyrate, preserves the IEC's mitochondria from ROS oxidative damage. A condition of dysbiosis increases mitochondrial damage, critically reducing the number of mitochondria but, above all modifying their morphology and permeability. A critical reduction in mitochondria leads to a decrease in the production of ATP by the IECs. A reduction in energy leads to a lower "hold" of the intercellular junctional complexes and an increase in bacterial translocation through the intestinal epithelium, which becomes more permeable. At the submucosal level, this condition increases inflammation and the recall of leukocytes, further worsening the condition of the mucosal barrier. Pathogenic mechanism of chronic intestinal diseases (CID), linked to the loss of impermeability and selectivity of the intestinal barrier, are induced by the action of TJs-released zonulin. Zonulin is a protein that modulates the permeability of TJs between cells of the intestinal barrier. Zonulin has been implicated in the pathogenesis of important GI diseases (i.e. coeliac disease and diabetes), and some glycoproteins, such as the gluten protein gliadin, activate zonulin signaling, increasing intestinal barrier permeability of macromolecules and contributing to "leaky gut" conditions. Thanks to the barrier effect, the condition of eubiosis, and the physiological traffic through the barrier of non-selfantigens, which are suitably presented to the leukocyte cells of the lamina propria (Th3, Tregs, etc.), there is the establishment of "oral tolerance" with the homeostasis of the GI mucosa. When environmental stimuli cause an imbalance of the microbiota, triggering the release of zonulin, loss of paracellular permeability, and an increase in the flow of antigens from the intestinal lumen to the lamina propria, antigens activate the immune system in a "pro-inflammatory" manner by causing the release of IFN-γ and TNF-α. This inflammation further exacerbates the increase in intestinal permeability and immune response, worsening and chronicizing the inflammation. This vicious circle, even more serious in genetically predisposed individuals, causes the interruption of oral tolerance to food antigens and causes the aggravation of chronic enteropathies.
