Monocytes, which originate from myeloid bone marrow precursors, constitute a component of the innate immune system. Upon maturation, they exit the bone marrow and travel through the bloodstream until they are recruited to eliminate dying cells and foreign antigens from damaged tissues. Within the tissue environment, they differentiate into macrophages, eventually returning to the peripheral circulation.
In peripheral blood, three distinct stages of monocyte maturation can be identified immunophenotypically:
Classical monocytes (CD14+CD16-), comprising roughly 80% of all monocytes.
Intermediate monocytes (CD14+CD16+).
Non-classical monocytes (CD14-/dim CD16+/++).