Monocytes are known to originate in the bone marrow from a common myeloid progenitor. Monocytes express various receptors, which monitor and sense environmental changes. Monocytes are highly plastic and heterogeneous, and change their functional phenotype in response to environmental stimulation. Evidence from murine and human studies has suggested that monocytosis can be an indicator of various inflammatory diseases. Monocytes can differentiate into inflammatory or anti-inflammatory subsets. Upon tissue damage or infection, monocytes are rapidly recruited to the tissue, where they can differentiate into tissue macrophages or dendritic cells.
Monocytes were initially identified by their expression of large amounts of CD14. However, the subsequent identification of differential expression of antigenic markers showed that monocytes in human peripheral blood are hetero geneous. CD16 is expressed on the surface of monocytes. Differential expression of CD14 and CD16 allowed monocytes to be divided into two subsets: CD14hiCD16–cells, which are often called classic monocytes, because this phenotype resembles the original description of monocytes; and CD14+CD16+ cells. Mouse blood monocytes are identified primarily by their expression of CD115, CD11b, and the F4/80 antigen. For example, human monocytes have also been classified based on their CD64 expression , as well as on expression of the adhesion molecule CD56. Additionally, CD68 is a very classic marker for human macrophages. Some people use CD71 to distinguish between monocytes and macrophages, because it is abundant in macrophages but minimally expressed in monocytes.