Figure 1.

Human monocyte subsets in health. Human monocytes mature in the bone marrow and are subsequently released into the circulation as CD14⁺ classical monocytes. Progressively, classical monocytes (CD14⁺CD16⁻) give rise to non-classical monocytes (CD14^dimCD16⁺) through an intermediate step of CD14⁺CD16⁺ monocytes. Classical monocytes in humans can be distinguished from the other two subsets by additional markers, such as CD36, CCR2, and CD64 and take part in the host's anti-microbial responses, such as adhesion to the endothelium, migration, and phagocytosis. Intermediate monocytes are characterized by their high expression of CCR5 and HLA-DR molecules and are involved in antigen processing and presentation and transendothelial migration. Non-classical monocytes divide into a SLAN⁺ and a SLAN⁻ population, express high levels of CX₃CR1 and specialize in complement and FcR-mediated phagocytosis, transendothelial migration and anti-viral responses. CM, classical monocytes; IM, intermediate monocytes; NCM, non-classical monocytes.