Figure 1

Classical activation (M1), alternative activation (M2) and deactivation of macrophages. Classical activation is mediated by the priming stimulus IFN-γ, followed by a microbial trigger (lipopolysaccharide, LPS). Alternative activation is mediated by IL-4 and IL-13, acting through a common receptor chain (IL-4Rα). Deactivation can be innate or acquired in origin. The uptake of apoptotic cells or lysosomal storage of host molecules generates anti-inflammatory responses. Cytokines (IL-10, TGF-β, M-CSF, IFNα/β) and glucocorticoids are potent modulators of activation. Pathogens can deactivate macrophages by various mechanisms.