The lung, to serve its primary function of gas exchange, is the organ with the largest epithelial surface area (approximately 70 m²) in continuous contact with the external environment.<sup>59 Murray J. The Normal Lung   Philadelphia: WB Saunders (1986). 

Cliccare qui per andare alla sezione Riferimenti</sup> As normal respiration occurs, the upper and lower airways of humans are repeatedly exposed to numerous airborne particles and microorganisms. Deposition of these foreign substances may occur on the surface of the upper or lower respiratory tract, and both nonspecific and specific defense mechanisms have evolved to eliminate these challenges. In the event of a breakdown of any component of these defenses, chances of infection of the upper or lower respiratory tract escalate. Because there are structural and functional differences between the conducting airways and the respiratory gas exchange airways, defense mechanisms vary by site in the respiratory system. Normal respiratory defense mechanisms are reviewed here, and implications for therapy for pulmonary infections are discussed.