The monoclonal antibody 2A1 recognizes rat C5b-9. The antibody was shown to compete with antibodies to human C9 for its binding site on the C5b-9 complex, indicating that the reactive epitope is located on the C9 molecule. C5b-9 membrane attack complexes are assembled from five precursor molecules in the serum. Proteolytic cleavage of C5 by C5 convertase generates C5b which initiates assembly of the C5b-9 complex. The last step of C5b-9 complex formation involves polymerization of C9 which accompanies insertion of the complex into the cell membrane. During formation of C5b-8 and C9 polymerization, neoantigens are generated which are unique to the C5b-9 complex and are not present on any of the individual native complex components. The complement regulatory proteins CD59 and complement S-protein can both prevent C5b-9 insertion into the cell membrane. The formed SC5b-9 complex is unable to attach to cells and is cytolytically inactive.C5b-9 is involved in the progression of chronic proteinuric renal disease by mediating continuous tubulointerstitial damage. Early tubulointerstitial injury in the remnant kidney can be improved when C5b-9 complex forming is abrogated.The monoclonal antibody 2A1 was raised against a rat C5b-9 neoantigen. Monoclonal antibody 2A1 can be used as a coating antibody to detect C5b-9 in plasma and urine samples.