Series introduction: Innate host defense of the respiratory epithelium.

This series overviews the current knowledge of the innate-immune system of the lung. Special emphasis is put on mechanisms used by epithelial cells of the respiratory tract to contribute to host defense. This defense function is important, as the lung is especially prone to microbial exposure. The exchange of oxygen and carbon dioxide requires the ventilation of a large volume of environmental air that may contain respiratory pathogens. Furthermore, the digestive pathway shares with the airways the pharynx, opening another possibility for the uptake of microorganisms into the respiratory tract. Host defense is provided by a multilayered and complex system that aims to avoid colonization and infection of the lung. A classic view separates the inborn, innate-immune system from adaptive mechanisms that involve antigen-specific lymphocytes.