How Monocytes Function in the Body

By Karen Raymaakers  – Updated on November 15, 2019

 Medically reviewed by Richard N. Fogoros, MD 

Monocytes are a type of white blood cell. Like other white blood cells, monocytes are important in the immune system’s ability to destroy invaders, but also in facilitating healing and repair. Monocytes are formed in the bone marrow and are released into peripheral blood, where they circulate for several days. They comprise about 5% to 10% of the circulating white blood cells in healthy individuals.

Monocytes are probably best known for their role in serving as something akin to reserve forces in the military. Some of them may be called up if needed, to form the precursors of two other types of white blood cells: tissue macrophages and dendritic cells. But monocytes also have other roles in infection and disease, some of which have nothing to do with tissue macrophages and dendritic cells.

Until recently, the main role of monocytes was considered to be sensing the environment and replenishing the pool of tissue macrophages and dendritic cells, as needed.

Now it is known that different subsets of monocytes have different markers or protein tags on the outside, and these subsets may also behave differently.1 Three different kinds of human monocytes are now described:

When it comes to the different kinds of monocytes and how they function in the immune system, researchers are still working out the details, and much more is currently known about mouse monocytes than human monocytes.

The terms “inflammatory” and “anti-inflammatory” are also used to describe human monocytes, based on the particular protein tags, or receptors, found on the outside of these cells. It is not yet certain in humans, however, what proportion of monocytes are mobile enough to go in and out of tissues, and evidence suggests there may be kinds of monocytes that can engulf and digest, or phagocytize, invaders but without actively promoting inflammation.

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