- Proper pulmonary ventilation requires a sterile environment to proceed normally, yet enormous volumes of external air potentially containing pathogen-laden droplets are ventilated daily. Furthermore, the outlet of the respiratory system is immediately adjacent to that of the GI Tract and the nasopharynx which a vibrant microbial flora calls home. Consequently, powerful mechanisms exist which prevent colonization of the respiratory system and maintain its sterility.
- Hair within the nostril impedes entry of large, potentially pathogen-laden particles from being inhaled into the lower respiratory tract. Particles and pathogens which do enter the conducting airways become entrapped in mucous secreted by goblet cells which is then moved out of the respiratory system by action of the ciliated respiratory epithelium (This is termed the mucociliary elevator). Coughing and the gag reflex help reduce aspiration of GI contents.
- Particles which do enter alveoli are immediately phagocytosed by alveolar macrophages which either kill the pathogen or remove them by riding the mucociliary elevator out of the lung. Additionally, proteins within surfactant display opsonizing properties, aiding in alveolar macrophage phagocytosis. If alveolar macrophages are overwhelmed, they may release inflammatory cytokines, such as IL-8 which recruit neutrophils to aid in microbial killing. Previous exposure to pathogens results in generation of IgA and IgG antibodies which are secreted in mucous and surfactant that aid in opsonization and bacterial clearance.