Monocytes are a population of leukocyte. They belong to the so called PBMC and are the largest type of cells in this population. Monocytes can differentiate into macrophages and myeloid lineage dendritic cells. These are professional APC´s. They are a part of the innate immune system but alos 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 produced by the bone marrow from precursors called monoblasts. Monocytes circulate in the bloodstream for about 1-3 days and then typically move into tissues throughout the body where they differentiate into macrophages and dendritic cells.
Monocytes and their macrophage and dendritic-cell derivates serve three main functions in the immune system. These are phagocytosis, antigen presentation, and cytokine production. Phagocytosis is the process of uptake of microbes and particles followed by digestion and destruction of this material. 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-dependent cell-mediated cytotoxicity. Vacuolization may be present in a cell that has recently phagocytized foreign matter.