Tracheobronchial secretions are a complex mixture of secretory fluids derived from sources within the lung. Important constituents include the mucous glycoproteins, other secretory proteins, serum proteins, lipids, salts; water makes up 95% of mucus by weight. These secretions form two phases at the epithelial surface: a mucous gel and an aqueous layer (periciliary fluid). Polymerization and aggregation of mucous glycoproteins create the gel matrix. Other macromolecules such as lysozyme, albumin, and immunoglobulin A also may participate in the process of gelation.