Figure 1. Monocyte and macrophage subsets.

Monocytes and macrophages are generally divided into two subsets that represent different expression patterns of surface markers and functions. Monocytes are divided into Ly6C^hi/CD14^hiCD16^– monocytes (50% in mice and 90% in humans, respectively) and Ly6C^low/CD14⁺CD16⁺ monocytes (50% in mice and 10% in humans, respectively). Ly6C^hi/CD14^hiCD16⁻ monocytes are characterized by expression of CCR2, which allows for trafficking of the monocytes towards inflamed tissues. Ly6C^low/CD14⁺CD16⁺ monocytes lack CCR2 but express CCR5 instead. It is not known if one of these subsets specifically matures into one of the two major macrophage subsets. M1 macrophages are known as inflammatory macrophages. They produce inflammatory cytokines and reactive oxygen species. They primarily stimulate the Th1 response and are involved in tissue destruction in many inflammatory diseases. M2 macrophages are described as anti-inflammatory or regulatory macrophages. They produce IL-10 to dampen local inflammation and are mainly involved in the Th2 response and chronic diseases. Their ability to produce tissue remodeling factors makes them valuable in the remodeling and regeneration of tissue. However, their functions are also a cause of unwanted tolerance in cancer.