Respiratory tract antimicrobial defense system

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.

Source: Wikipedia — Respiratory tract antimicrobial defense system (CC BY-SA 4.0)