CD55 is the complement regulatory protein, decay accelerating factor (DAF). It is a 70kD glycoprotein (in erythrocytes) anchored in the membrane by glycosylphosphatidylinositol tail. In other cells the apparent molecular weight is somewhat larger. It has a substantial content of O-glycans, and also on N-glycan. DAF binds to activated C4b or C3b complement fragments on the cell surface, preventing the assembly and accelerating the decay of both classical and alternative pathways. DAF carries the Cromer related blood group antigens. DAF has a wide distribution on cells in non-haemopoietic tissues, particularly epithelium and is specifically found at the foetal-maternal interface in placenta. Soluble forms of DAF are found, for example, in plasma, saliva and urine. The antigen on erythrocytes is pronase and chymotrypsin sensitive, but resistant to trypsin.Synonyms: Complement decay-accelerating factor