Monocyte is a type of white blood cell, part of the human body's immune system.
Monocytes have two main functions in the immune system: (1) replenish resident macrophages and dendritic cells under normal states, and (2) in response to inflammation signals, monocytes can move quickly (approx. 8-12 hours) to sites of infection in the tissues and divide/differentiate into macrophages and dendritic cells to elicit an immune response. Monocytes are usually identified in stained smears by their large bilobate nucleus.
Monocytes are produced by the bone marrow from haematopoietic 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 constitute between three to eight percent of the leukocytes in the blood. In the tissues monocytes mature into different types of macrophages at different anatomical locations.
Monocytes are responsible for phagocytosis (ingestion) of foreign substances in the body. Monocytes can perform phagocytosis using intermediary (opsonising) proteins such as antibodies or complement that coat the pathogen, as well as by binding to the microbe directly via pattern-recognition receptors that recognize pathogens. Monocytes are also capable of killing infected host cells via antibody, termed antibody-mediated cellular cytotoxicity. Vacuolization may be present in a cell that has recently phagocytized foreign matter.
Monocytes which migrate from the bloodstream to other tissues will then differentiate into tissue resident macrophages or dendritic cells. Macrophages are responsible for protecting tissues from foreign substances but are also suspected to be the predominant cells involved in triggering atherosclerosis. They are cells that possess a large smooth nucleus, a large area of cytoplasm and many internal vesicles for processing foreign material.
Monocyte Subpopulations :
There are two types of monocytes in human blood:
- the classical monocyte, which is characterized by high level expression of the CD14 cell surface receptor (CD14++ monocyte)
- the non-classical, pro-inflammatory monocyte with low level expression of CD14 and with additional co-expression of the CD16 receptor (CD14+CD16+ monocyte).
The CD14+CD16+ monocytes develop from the CD14++ monocytes, i.e. they are a more mature version. After stimulation with microbial products the CD14+CD16+ monocytes produce high amounts of pro-inflammatory cytokines like tumor necrosis factor and interleukin-12.