Your small intestine has a very thin layer of cells constructing its walls and creating its border. These are called epithelial cells. They are equipped with tight junctions that allow them to stay tightly connected with each other, forming a bridge that only lets very small molecules pass through.
These cells are protected by a layer of mucous in the small intestines called the mucosal layer. Normally the mucous is full of beneficial bacteria that provide protection against undigested food, toxins, and parasites as well as offer constant nourishment to the epithelial cells, otherwise keeping us healthy.
Leaky gut occurs when there is a breakdown of the mucosal layer and/or damage to the epithelial cells. This is often caused by poor dietary choices, antinutrients in foods, pathogens, toxins, antibiotic and drug use, and stress. What is supposed to stay in the gut starts to travel across the intestinal barrier into the bloodstream.
All autoimmune conditions, food intolerances, allergies, and chronic inflammation have direct connections to a leaky gut.