The 3-C39 monoclonal antibody reacts with human CD14. CD14 is a 53-55kDa cell surface glycoprotein that is expressed by mature monocytes, macrophages, and activated granulocytes as a glycophosphatidylinositol-linked protein. CD14 functions as a receptor for lipopolysaccharide (LPS, endotoxin) when LPS is bound to LBP (LPS binding protein). Soluble forms of CD14 (sCD14) can be detected either in serum or in supernatants from cell cultures. Soluble CD14 is produced either by shedding of membrane CD14 or by release from cells before addition of the GPI anchor. Both membrane and sCD14 function to enhance cell activation by LPS. Binding of LPS-LBP complexes to CD14 activates monocytes or macrophages to produce and secrete cytokines and pro-inflammatory mediators. Non-CD14 expressing cells, such as endothelial cells, can be activated by a complex of LPS and sCD14. Serum levels of sCD14 increase in association with sepsis, infectious disease, autoimmunity and allergic disorders.