The respiratory tract antimicrobial defense system is a layered defense mechanism which relies on components of both the innate and adaptive immune systems to protect the lungs and the rest of the respiratory tract against inhaled microorganisms.
In the first line of defense, inhaled bacteria are trapped by mucus and are swept toward the pharynx and are swallowed.[1] Bacteria which penetrate the mucous layer are dealt with a second line of defense which includes antimicrobial peptides that are secreted by the surface epithelium of the respiratory tract which kill many strains of bacteria.[2] Those bacteria that are resistant to antimicrobial peptides are killed by a variety of reactive oxygen species produced by phagocytes. In a third line of defense and as a last resort, persistent bacterial infections which escape the innate immune system are eliminated by the adaptive immune system.