The many faces of macrophage activation

It used to be easy. In the old days (∼8 years ago), activated macrophages were simply defined as cells that secreted inflammatory mediators and killed intracellular pathogens. Things are becoming progressively more complicated in the world of leukocyte biology. Activated macrophages may be a more heterogenous group of cells than originally appreciated, with different physiologies and performing distinct immunological functions. The first hint of this heterogeneity came with the characterization of the “alternatively activated macrophage” [1]. The exposure of macrophages to interleukin (IL)-4 or glucocorticoids induced a population of cells that up-regulated certain phagocytic receptors but failed to produce…