External factors (particularly the consumption of ultra-processed foods, but also other dietary factors, infections, inflammatory bowel diseases etc.)can influence both the gut microbiota and epithelial barriers leading to a pro-inflammatory milieu: when the epithelial barrier is damaged, antigens may cross freely, causing the release of pro-inflammatory epithelial-derived molecules like IL-25, IL-33 and thymic stromal lymphopoietin (TSLP), which promote T-naïve cell differentiation into Th2 cells, IgE class-switching, and tissue accumulation of mast cells and eosinophils (124). Here, TSLP is linked to inflammatory bowel disease.