ABSTRACT
I. Introduction The epithelium of the lung is the largest surface area of the body and is exposed to gaseous compounds and numerous inhaled particles present in inhaled air that have escaped the filtering function of the nose and have not deposited in the nasopharynx. These particles include respiratory pathogens that may cause severe infections resulting in substantial morbidity and mortality. Nevertheless, the peripheral small airways and alveoli are sterile in healthy subjects and severe infections are relatively rare in such individuals. This is explained by the presence of an efficient host-defense system in the lung. A breach in this defense or aberrant functioning of the system results in infectious and inflammatory lung disease. Intense research has led to the characterization of the various elements that constitute this defense system, and has provided new insight into the pathogenesis of a variety of infectious and inflammatory lung disorders. Host defense in the lung includes the barrier function and mucociliary clearance provided by the epithelium, as well as innate and adaptive immune responses mediated by a range of cell types in the lung, including epithelial cells, phagocytes, dendritic cells (DCs), and lymphocytes. In this chapter, an overview is provided of the various elements of this defense system.
