CD55, also known as decay-accelerating factor (DAF), is a GPI-anchored membrane inhibitor of complement. It inhibits complement activation by interfering with the function of C3 and C5 convertases in both the classical and alternative pathways. CD55 is expressed on most cells to protect them from complement-mediated attack. CD55 and other membrane complement regulatory proteins (CD46 and CD59) are frequently overexpressed on cancer cells, possibly as a mechanism for cancer cells to overcome lysis by complement. Virus-receptor interactions have been elucidated for several viruses that use CD55 as an adhesion or invasion receptor, including enterovirus, echovirus, and coxsackie B virus. Moreover, CD55 also interacts with an epidermal growth factor-seven-span transmembrane (EGF-TM7) receptor, CD97. CD97 antigen becomes immediately up-regulated on most leukocytes during activation and is constitutively expressed by macrophages, dendritic cells, and some epithelial tumors.Synonyms: Complement decay-accelerating factor