 Some of the proteins in this group are responsible for the molecular basis of the blood group antigens, surface markers on the outside of the red blood cell membrane.Most of these markers are proteins, but some are carbohydrates attached to lipids or proteins.Complement decay-accelerating factor (Antigen CD55) belongs to the Cromer blood group system and is associated with Cr(a), Dr(a), Es(a), Tc(a/b/c), Wd(a), WES(a/b), IFC and UMC antigens.