[5] Monocytes circulate in the bloodstream for about one to three days and then typically migrate into tissues throughout the body where they differentiate into macrophages and dendritic cells.

[6][7] Today, three types of monocytes are recognized in human blood:[8] While in humans the level of CD14 expression can be used to differentiate non-classical and intermediate monocytes, the slan (6-Sulfo LacNAc) cell surface marker was shown to give an unequivocal separation of the two cell types.

[10][11] Ghattas et al. state that the "intermediate" monocyte population is likely to be a unique subpopulation of monocytes, as opposed to a developmental step, due to their comparatively high expression of surface receptors involved in reparative processes (including vascular endothelial growth factor receptors type 1 and 2, CXCR4, and Tie-2) as well as evidence that the "intermediate" subset is specifically enriched in the bone marrow.

Resident monocytes have the ability to patrol along the endothelium wall in the steady state and under inflammatory conditions.

[13][14][15][16] Monocytes are mechanically active cells[17] and migrate from blood to an inflammatory site to perform their functions.

As explained before, they can differentiate into macrophages and dendritic cells, but the different monocyte subpopulations can also exert specific functions on their own.

In general, monocytes and their macrophage and dendritic cell progeny serve three main functions in the immune system.

[18] In vitro, monocytes can differentiate into dendritic cells by adding the cytokines granulocyte macrophage colony-stimulating factor (GM-CSF) and interleukin 4.

Non-classical monocytes produce high amounts of pro-inflammatory cytokines like tumor necrosis factor and interleukin-12 after stimulation with microbial products.

[30] In the field of atherosclerosis, high numbers of the CD14++CD16+ intermediate monocytes were shown to be predictive of cardiovascular events in populations at risk.

[36] Also, non-classical slan+ monocytes are strongly reduced in patients with hereditary diffuse leukoencephalopathy with spheroids, a neurologic disease associated with mutations in the macrophage colony-stimulating factor receptor gene.