Figure 1.

A snapshot of monocyte and macrophage diversity. Two main phenotypically distinct subsets can be identified in the blood: inflammatory monocytes (CCR2⁺Ly6C⁺ in mice; CCR2⁺CD14⁺CD16⁻ in humans) and patrolling monocytes (CX3CR1⁺ in mice; CX3CR1⁺CD14^+/−CD16⁺ in humans). In tissues, macrophages in different organs have different morphological and functional features (e.g., peritoneal macrophages, alveolar macrophages, and liver Kupffer cells). Upon activation with specific signal, macrophages initiate functional programs that are dictated by transcription factors (in rectangles). Two main functional polarizations can be distinguished: classical or M1 and alternative or M2. Other signals, including immune complexes in conjunction with LPS or IL-1, and immune-suppressive cytokines, including IL-10 and TGFβ, also turn on macrophages along an M2-like polarization.