CELLS

Monocytes are a type of leukocytes that develop from long-term hematopoietic stem cells in the bone marrow.^1,2

Monocytes can be classified as either classical inflammatory monocytes, nonclassical endothelial patrolling monocytes, or intermediate monocytes. Classical monocytes are released into the circulation in a CCR2-dependent manner and localize to the site of infection or injury via a chemokine gradient. Nonclassical monocytes are involved in intraluminal surveillance of endothelium and phagocytosis of injured endothelium along with recruitment of neutrophils to the site of injury. Intermediate monocytes are shown to have higher MHC II expression and are more closely related to classical monocytes with intermediate expression of the surface markers.¹

Monocytes are characterized by their ability to recognize “danger signals” via pattern-recognition receptors.¹ Once recruited to tissues, monocytes can differentiate into macrophages and DCs.^1,2 Monocyte functions include phagocytosis and presentation of antigens, secretion of chemokines, and proliferation in response to infection and injury.¹

Macrophages are a type of white blood cell that are produced through the differentiation of monocytes and make up 20% of the PBMCs.³

Different populations (M1, M2, and M3) of macrophages with distinct physiological features have been identified. Switching from the M1 to the M2 phenotype protects the organism from excessive inflammation, whereas switching from the M2 to the M1 phenotype protects from allergic and asthmatic Th2 reactions and a decrease in bactericidal properties of macrophages and favors the resolution of inflammation. Pathogenesis of inflammatory diseases could be associated with a disturbance in the formation of the M3 switching phenotype.³

Macrophages are uniquely equipped to detect and respond to tissue invasion by infectious organisms and to tissue injury through various scavengers, pattern-recognition receptors, and phagocytic receptors. They can also participate in removal of dead cells and cell debris while also contributing to tissue repair and recovery from injury and lesion.³

Both monocytes and macrophages are members of the mononuclear phagocyte system, a component of innate immunity.¹

CCR2: C-C chemokine receptor type 2; CD: cluster of differentiation; DC: dendritic cell; PBMC: peripheral blood mononuclear cell; MHC: major histocompatibility complex; Th: T helper.