Monocyte

Monocytes are a type of white blood cell, part of the human body's immune system. They are usually identified in stained smears by their large two-lobed nuclei. They are a kind of reserve cell which turn into macrophages and immune helper cells called dendritic cells.

Monocyte under a light microscope (40x) from a peripheral blood smear surrounded by red blood cells.

Monocytes work at two speeds in the immune system:

1. To gradually replenish resident macrophages and dendritic cells under normal conditions, and
2. To move quickly (~ 8-12 hours) to infected tissue in response to inflammation signals. There they divide and differentiate into macrophages and dendritic cells to cause an immune response.

Half of all monocytes are stored as a reserve in the spleen;^[1] the rest are circulating or in tissues.

Physiology

Monocyte

Monocytes are produced by the bone marrow from stem cell precursors called monoblasts. Monocytes circulate in the bloodstream for about one to three days and then typically move into tissues throughout the body. They make up three to eight percent of the leukocytes in the blood.

Monocytes which migrate from the bloodstream to the tissues will then differentiate into macrophages or dendritic cells, which then stay in the tissue. Macrophages are responsible for protecting tissues from foreign substances. They are cells that possess a large smooth nucleus, a large area of cytoplasm and many internal vesicles for processing foreign material.

Monocytes and their macrophage and dendritic cell progeny serve three main functions in the immune system. These are phagocytosis, antigen presentation and cytokine production.^[2]

1. Phagocytosis is the process of uptake of microbes and particles followed by digestion and destruction of this material. Monocytes are also capable of killing infected host cells using antibodies.
2. Microbial fragments that remain after such digestion can serve as antigens. This process is called antigen presentation and it leads to activation of T lymphocytes. These mount a specific immune response against the antigen.
3. Other microbial products can directly activate monocytes and this leads to the production of pro-inflammatory cytokines.

References

1. Swirski F.K. et al. 2009. Identification of splenic reservoir monocytes and their deployment to inflammatory sites. Science, 325: 612-616.
2. Cytokines are small molecules which carry local messages between cells.