Mucus as a natural barrier.

The airway mucus varies in amount, composition and physical properties at different levels of the airways and thus variation in types and/or degree of defence are expected. The mucus barrier, that is the barrier provided by the secretory products from the mucous membrane, comprises three compartments: the epiphase (gel layer); the hypophase (sol layer), and the glycocalyx. The epiphase is the best characterized of the three and, because of its direct contact with the external environment, is the more complex and can act as a physical, chemical and biological barrier. The hypophase and the glycocalyx barriers are less well known in terms of composition, origin, control and functions. The periciliary/sol layer, apart from providing a suitable environment for the cilia, can further dilute substances which have crossed the gel layer. The glycocalyx represents the last protective barrier for the cell membrane and is likely to play an important role in the defence of the surface epithelium. A better understanding of their origin, composition and control, both in health and disease, will help us to choose the appropriate therapeutic agent for enhancing their efficacy as natural barriers.