Macrophages : identification, separation, and function.

Within the human lung, macrophages can be found in the pleura, interstitium, alveoli, airways, vasculature, and walls of the bronchi and bronchiols. This distribution does not simply reflect the ubiquitous nature of these cells, as the macrophages found at these different sites show subtle distinctions in terms of cell physiology and phenotype (1). Further, animal studies have revealed functional differences between macrophages from different lung compartments (2). These differences may however be more apparent than real. Macrophages are motile cells and those, for example, present in the airways may arrive via the lung interstitium and are known to be capable of migrating back into the tissues of the lung. Thus, any observed differences between cells in different compartments are likely to be a reflection of the particular environment the cells find themselves in, rather than definitive distinctions between cell types (reviewed in 3). The message is that macrophages are "plastic" in terms of their phenotype. As different phenotypes have been shown to reflect different functions, it would seem inevitable, therefore, that these cells also exhibit a diversity of function. Indeed, it is now recognized that the phagocytic scavenger or microbicidal effector cell, are just two of several roles these cells can play.