[Physiological roles of the respiratory epithelium]

The epithelium of the airways plays a complex role in the organism's defence system against aggressive inhaled agents. Classically, the continuous cell layer of the epithelial mucosa acts as a protective barrier, but physiological role is much more complex and varied. The airway epithelium transports secretions by a collaborative effect of the ciliary movements and the rheological properties of the mucosa resulting from hydro-electrolytic movements occurring transepithelially. The epithelium participates in bronchial inflammatory response by interactions with inflammatory cells in the mucosal itself and by its capacity to produce free radicals and inflammatory mediators. Its membrane enzymes also participate in regulating neurogenic inflammation. A further role involves repairing damage which could compromise mucosal integrity. The airway epithelium participates in the immune defence system by interacting with lymphocytes and probably also by its capacity to express class II HLA molecules responsible for antigen presentation. Finally, the epithelium participates in anti-infectious mechanisms via its micro-organism receptors, its relationship with immune cells and inflammatory cells, its participation in mucociliary clearance and finally by the secretion of antiinfectious molecules (lysozymes, free radicals) by the mucosa.