Pulmonary host defense mechanisms.

T he major function of the respiratory system is to procure oxygen from the external environment and to eliminate carbon dioxide from the body at rates required by tissue metabolism. Unfortunately, ambient inspired air contains not only oxygen which is vital to survival, but also noxious gases and a multitude of particulates, including viable microorganisms, which the lungs must exclude to maintain good health. Toxic substances in air are derived from many sources. For example, naturally occurring processes can give rise to large amounts of aerosolized particles. The latter includes resuspension ofsoil and emissions from volcanic activity, forest fires, photochemical reactions, and even sea spray. Airborne particles also include biologically derived viruses, bacteria, fungi, algae, spores, and pollens. In addition, industrial and occupational sources add sulfur dioxide, carbon monoxide, nitrogen dioxide, ammonia, hydrocarbons, ozone, and inorganic and organic particles to our inspired air. This article will briefly overview the various pulmonary defense mechanisms which usually prevent the development of lung disease by resisting invasion by pathogenic organisms and by absorbing, detoxifying, and/or removing toxic gases and inanimate particulates. Although the respiratory tract defenses encompass many interrelated responses, for simplicity of presentation, we have arbitrarily divided them into two major parts (Table 1). One is referred to as physical defense mechanisms which primarily comprise structural and physiochemical adaptations in the proximal upper and lower airways and their lining layers. The second is cellular defense mechanisms which operate mostly in the distal alveolar parts ofthe lung. The latter largely depends on alveolar macrophages (AM) and their ability to mediate phagocytic and immunologic responses. Finally, although the aforementioned prox-