Anti-CD55 antibodies are used in the immunodetection of the protein CD55 molecule (Cromer blood group). In humans, the canonical protein has a reported length of 381 amino acid residues and a mass of 41.4 kDa. Up to 7 different isoforms have been reported for this protein. It is noted to be expressed on the plasma membranes of all cell types that are in intimate contact with plasma complement proteins. A member of the Receptors of complement activation (RCA) protein family, CD55 is known to recognize C4b and C3b fragments that condense with cell-surface hydroxyl or amino groups when nascent C4b and C3b are locally generated during C4 and c3 activation. Post-translational modifications have been described, including O-glycosylation. The CD55 gene has been associated with the disease, Complement hyperactivation, angiopathic thrombosis, and protein-losing enteropathy. Synonyms for this target antigen include CR, DAF, TC, complement decay-accelerating factor, and CHAPLE. CD55 gene orthologs have been reported in the rat, bovine and chimpanzee species.