CD14 is a 55-kDa protein found as a glycosylphosphatidylinositol (GPI)- anchored protein on the surface of monocytes, macrophages, and polymorphonuclear leukocytes, and as a soluble protein in the blood. Its main function is to serve as a receptor for lipopolysaccharide (LPS). Besides its role in endotoxin signaling, it has been proposed that CD14 is involved in the transportation of other lipids, cell-cell interactions during different immune responses, and recognition of apoptotic cells. CD14 is highly expressed on the surface of monocytes/macrophages and strongly up-regulated during the differentiation of monocytic precursor cells into mature monocytes. Therefore, CD14 has been commonly used as a differentiation marker for monocytes/macrophages. An antibody to CD14 also labels Langerhans' cells and dendritic cells.