Monocytes

Monocytes are a population of leukocytes. They belong to the so-called PBMC and are the largest cell type in this population. Monocytes can differentiate into macrophages and myeloid lineages of dendritic cells. These are professional antigen presenting cells (APC). They are part of the innate immune system, but are also very important for triggering the process of adaptive immunity. There are at least three subclasses of monocytes in human blood, based on their phenotypic receptors.

Monocytes are formed by the bone marrow from precursors known as monoblasts. Monocytes circulate in the bloodstream for about 1-3 days and then typically migrate to tissues throughout the body, where they differentiate into macrophages and dendritic cells.

Monocytes and their macrophage and dendritic cell derivatives fulfill three main functions in the immune system. These are phagocytosis, antigen presentation and cytokine production. Phagocytosis is the process of ingestion of microbes and particles, followed by digestion and destruction of this material. Monocytes can perform phagocytosis with the help of intermediate (opsonizing) proteins such as antibodies or complements that envelop the pathogen, as well as by direct binding to the microbe via pattern recognition receptors that recognize pathogens. Monocytes are also able to kill infected host cells by antibody-dependent, cell-mediated cytotoxicity. Vacuolization may be present in a cell that has recently phagocytosed foreign substances.