The monoclonal antibody TH9 recognizes rat CD59. Rat CD59 was previously known as rat inhibitory protein (RIP) and it is a potent inhibitor of the complement membrane attack complex (MAC) action. CD59 regulates the formation and function of the lytic C5b-9 complex by binding C8 and preventing the unfolding and membrane insertion of C9 and by binding C9 and restricting its polymerization.CD59 is a small (18 - 25 kDa) molecule, linked to the cell membrane through a glycosyl phosphatidylinositol (GPI) anchor and comprising 77 amino acids with a single N-linked carbohydrate group at Asn-18. Analogues of CD59 can be found in all species with similar structures and sizes.In rat, CD59 is expressed on vacular endothelium and circulating cells. In the central nervous system (CNS) of the rat, CD59 is expressed on the Schwann sheath of peripheral nerve fibres and on ependymal cells, but not on glial cells and neurons in the CNS. Rat astrocytes in vitro express CD59 on its surface.CD59 may be involved in rheumatoid arthritis, motor nerve injury in the Guillain-Barré sydrome and in other diseases where defective inhibition of complement activation on self tissue is involved. Furthermore, CD59 may play an important part in abrogating the effects of complement attack in renal disease. Its presence and protective effect have already been demonstrated on human renal cells.