ABSTRACT
Epithelial cells lining mucosal surfaces essentially define the border between the environment and ourselves. At some mucosal surfaces, the epithelial border is only one cell thick, interfacing with the environment over huge surface areas. The mucosal immune system cannot be understood without understanding the epithelial cell and the developmental and molecular biology that underlie its involvement in mucosal physiology and immunology. The epithelial barrier of mucosal surfaces separates a wealth of foreign antigens that consist of the commensal flora and ingested food, on the luminal side, and the mostly sterile environment harboring the mucosal immune system on the other side. Each of the “wet” epithelial surfaces that line mucosal tissues possesses the common feature of separating the internal sterile milieus of the host from the external environment. The mucosal surfaces that support solute, ion, water, and gas transport are typically lined by a delicate single layer of epithelial cells, i.e., a simple epithelium.
