The respiratory epithelium is lined by mucus, a gel consisting of water, ions, proteins, and
macromolecules. The major macromolecular components of mucus are the mucin
glycoproteins, which are critical for local defense of the airway. There are three classes of
mucins in the airways: those that are secreted but do not polymerize (MUC7), those that are
secreted and polymerize to form gels (MUC5AC, MUC5B), and those that have
transmembrane domains and are cell surface associated (MUC1, MUC4, MUC16, MUC20) …