The chemokine system in diverse forms of macrophage activation and polarization.

Plasticity and functional polarization are hallmarks of the mononuclear phagocyte system. Here we review emerging key properties of different forms of macrophage activation and polarization (M1, M2a, M2b, M2c), which represent extremes of a continuum. In particular, recent evidence suggests that differential modulation of the chemokine system integrates polarized macrophages in pathways of resistance to, or promotion of, microbial pathogens and tumors, or immunoregulation, tissue repair and remodeling.