Some non-mucin components of mucus and their possible biological roles.

Non-mucin components have essential roles in the protective functions of mucous secretions. Secretory IgA (SIgA) antibodies probably act by blocking the attachment of pathogenic microorganisms to mucosal cells. In addition SIgA1 may render bacteria more 'mucophilic', possibly by virtue of the 'mucus-like' stretch that the immunoglobulin molecule possesses. Lysozyme will attack cell walls of susceptible bacteria. As the enzyme associates strongly with mucus glycoproteins the mucus layer is provided with powerful bactericidal properties. Lactoferrin, normally unsaturated, sequesters any free iron in secretions, so exerting a bacteriostatic action on iron-requiring microorganisms. In addition it may protect mucus glycoproteins from iron-catalysed active oxygen species. This mucoprotective action would be overcome during infections. Attention is also directed towards a possible copper-mediated limited degradation by hydrogen peroxide. Surfactants and free lipid have long been recognized as components of normal bronchial mucus. For example, some lipid is tightly but non-covalently bound to a hydrophobic region of bronchial mucin. More intriguing is the presence of small amounts of covalently bound lipid in normal human gastric mucin. In addition, normal human gastric mucus contains significant amounts of a galactose-rich polysaccharide. The function of this is not known but it may act as a cross-linking strand in the mucus gel structure or as a renewable cell membrane component, perhaps interacting between glycocalyx and the mucus layer.