CD55 also known as DAF is a glycosylphosphatidylinositol (GPI)-anchored single-chain glycoprotein that plays a crucial role in regulating the complement system. By inhibiting the formation of the C3 convertase enzyme complex, CD55 prevents complement-mediated lysis of cells, thereby protecting them from damage during immune responses. This protective function is particularly important in tissues that are in contact with serum, as CD55 is expressed on a variety of cell types, including hematopoietic and many non-hematopoietic cells. CD55′s ability to modulate the complement cascade highlights its significance in maintaining cellular integrity and preventing excessive inflammation, making CD55 a vital target for research in immunology and therapeutic applications.