CD55, also known as decay accelerating factor (DAF), is a 64-73kD GPI-anchored single chain glycoprotein that is expressed on all cells in contact with serum, including all hematopoietic cells and the vascular endothelium. A soluble form of CD55 is present in plasma and body fluids. CD55 is a member of the regulator of complement activity (RCA) family of proteins and is thought to play a role in the protection of cells from damage by autologous complement by preventing the amplification steps of the complement cascade. CD55 expression increases upon T cell activation and antibodies to CD55 are mitogenic in the presence of phorbol esters.1-5