Monocyte

Monocytes are a type of leukocyte or white blood cell. They are the largest type of leukocyte in blood and can differentiate into macrophages and monocyte derived dendritic cells. As a part of the vertebrate innate immune system monocytes also influence adaptive immune responses and exert tissue repair functions. There are at least three subclasses of monocytes in human blood based on their phenotypic receptors. Monocytes are amoeboid in appearance, and have nongranulated cytoplasm. Thus they are classified as agranulocytes, although they might occasionally display some azurophil granules and/or vacuoles. With a diameter of 15–22 μm, monocytes are the largest cell type in peripheral blood. Monocytes are mononuclear cells and the ellipsoidal nucleus is often lobulated/indented, causing a bean-shaped or kidney-shaped appearance. Monocytes compose 2% to 10% of all leukocytes in the human body. Monocytes are produced by the bone marrow from precursors called monoblasts, bipotent cells that differentiated from hematopoietic stem cells. Monocytes circulate in the bloodstream for about one to three days and then typically migrate into tissues throughout the body where they differentiate into macrophages and dendritic cells. The first clear description of monocyte subsets by flow cytometry dates back to the late 1980s, when a population of CD16-positive monocytes was described. Today, three types of monocytes are recognized in human blood: The classical monocyte is characterized by high level expression of the CD14 cell surface receptor (CD14++ CD16− monocyte) The non-classical monocyte shows low level expression of CD14 and additional co-expression of the CD16 receptor (CD14+CD16++ monocyte). The intermediate monocyte expresses high levels of CD14 and low levels of CD16 (CD14++CD16+ monocytes). While in humans the level of CD14 expression can be used to differentiate non-classical and intermediate monocytes, the slan (6-Sulfo LacNAc) cell surface marker was shown to give an unequivocal separation of the two cell types.