Monocytes

Monocytes are immune cells produced in bone marrow that enter the bloodstream and contribute to innate immunity. They belong to the mononuclear phagocyte system, a group of cells involved in detecting threats and engulfing material. Their roles include responses to infection, inflammation, and tissue injury .¹

Monocytes are a varied population. In humans, they are commonly divided into classical, intermediate, and non-classical groups using differences in proteins on their surfaces. These groups differ in their behavior, including recruitment to inflamed tissue and surveillance of blood vessels. Their activities depend on signals from their surroundings .¹

During inflammation, circulating monocytes can enter tissues and develop into macrophages, which help defend and maintain those tissues. However, many resident macrophage populations originate before birth and can maintain themselves locally. Monocytes are therefore an important source of macrophages in some settings, but they are not the continuing source of every macrophage in the body .¹