Macrophages in innate and acquired immunity.

Alveolar macrophages play a central role in pulmonary host defense. When foreign particles or pathogens enter the respiratory tract, constitutively present innate host defenses attempt to clear the challenge. Alveolar macrophage phagocytosis of foreign material is a critical component of this response, as is secretion of inflammatory mediators designed to combat invading pathogens. If the pathogenic burden is too large and overwhelms innate immunity, then acquired immune responses are initiated resulting in the generation of antigen-specific cellular and humoral immunity. In response to evolutionary pressures to minimize unnecessary inflammation in the lower respiratory tract, alveolar macrophages are generally poor accessory cells in the initiation of specific immunity. However, in many circumstances, especially those associated with cellular activation, alveolar macrophages can play an important role in the generation and expansion of pulmonary immune responses. This review discusses the role of alveolar macrophages in innate and acquired pulmonary host defense.