The small intestine: barrier, permeability and microbiota. Key messages for clinicians.

In recent years, there has been growing interest in the comprehension of the physiology of intestinal permeability and microbiota and how these elements could influence the pathogenesis of diseases. The term intestinal permeability describes all the processes that allow the passage of molecules as water, electrolytes and nutrients through the intestinal barrier by the para-cellular or the trans-cellular transport systems with several implications for self-tolerance and not-self immunity. An increased permeability might induce a more significant interaction of the immune system with unknown external antigens. This might favour the onset of several immune-related extra-intestinal diseases including coeliac disease, diabetes mellitus type 1, bronchial asthma and inflammatory bowel diseases. Furthermore, the intestinal permeability interacts every day with microbiota, the complex system of mutualistic inhabitants and commensal microorganisms living in the healthy gut. Microbiota is implicated in physiological functions by actively participating in digestion, absorption, synthesis of vitamins and protection from external aggressions. The critical site where these processes occur is the small intestine to which this updated review is dedicated. Understanding its anatomy, its barrier structure and permeability modulation and its microbiota composition is the essential skill to comprehend the complex pathogenesis of several - not only gastroenterological - diseases.