ABSTRACT
The primary function of the lungs is to perform gas exchange with the atmosphere. For this purpose, a great degree of complexity is required to facilitate the diffusion of oxygen and carbon dioxide while maintaining homeostasis. Branching airways terminate in as many as 300 million alveoli, resulting in the largest epithelial surface area of the body (1). This surface area consists of an alveolar-capillary interface, which is estimated to provide 50-100 m2 for gas diffusion (2). Therefore, the constituents of inspired air have extensive contact with the epithelial surface of the lung. This contact predisposes the body to many harmful agents such as dust, pollen, bacteria, and other micro-organisms, making the lung a uniquely susceptible portal for infection. Accordingly, the respiratory tract is equipped with numerous defense mechanisms that enable it to neutralize potential pathogens. These mechanisms begin in the nose with anatomic barriers such as nasal vibrissae
and extend to the alveoli where resident phagocytic cells constitute the first line of defense at the site of gas exchange.
