Figure 1 Steady-state homeostasis and macrophage polarization. Circulating monocytes after migrating to the tissues differentiate into macrophages upon encountering pathogens. Tissue macrophages can either undergo classical activation thereby promoting the M1 typeor alternative activation which promotes the M2 type. The pathway thatwill be adopted by the tissue macrophages is determined by the cytokine secretion profile. An equilibrium may exist between the M1 and M2 macrophages. Adapted from Guha et al.