The barrier components of gastrointestinal (GI)epithelia include an epithelium-adapted microbiomeinteracting with an outer, continuously shedding mechanical barrier, the latter being mucus, cornified cells,or both. The mentioned barrier components protect the living epithelial cells from mechanical damages and act together with immunological (e.g., immune cells,antibodies) and chemical (e.g., pH, defensins) barrier components to restrict microbial, especially pathogen,invasion.