The epithelial barrier has classically been considered as the only first line of defense against irritants, pathogens, and allergens. However, it is now known to play an essential role in the immune response to exogenous agents. In fact, recent reports postulate the epithelial barrier hypothesis as a possible explanation for the increasing incidence and severity of allergic diseases.
The epithelial barrier preserves the isolation of internal tissues from potential external threats. Moreover, a coordinated interaction between epithelial and immune cells ensures the unique immune response taking place in mucosal tissues, which is reported to be dysregulated in allergic diseases.
We and others have demonstrated that in severe allergic phenotypes, the epithelial barrier undergoes several histological modifications, with increased infiltration of immune cells, leading to dysfunction. This is common in atopic dermatitis, asthma, and food allergy. However, the precise role of the epithelial barrier in mucosal biology during progression of allergic diseases is not well understood.
In this review, we aim to compile recent knowledge regarding the histological structure and immunological function of the epithelial barrier and to shed light on the role of this compartment in the onset and progression of allergic diseases.

Key words: Epithelium, Mucosal immunity, Allergy, Antigen, Immunoglobulin (Ig) switch, Inflammation