Airway secretions actively participate in respiratory epithelium protection. Apart from its main participation in transport of inhaled microorganisms and particles by mucociliary clearance, respiratory mucus also contributes to other protective purposes such as the control of airway humidification. Biochemical components found in secretions, such as mucins, lipids, antibacterial agents (secretory IgA, lysozyme, lactoferrin), antioxidant and antiprotease components, contribute significantly to the airway epithelium defense.