Potent inhibitor of the complement membrane attack complex (MAC) action, which protects human cells from damage during complement activation (PubMed:11882685, PubMed:1698710, PubMed:2475111, PubMed:2475570, PubMed:2606909, PubMed:9053451). Acts by binding to the beta-haipins of C8 (C8A and C8B) components of the assembling MAC, forming an intermolecular beta-sheet that prevents incorporation of the multiple copies of C9 required for complete formation of the osmolytic pore (PubMed:11882685, PubMed:1698710, PubMed:36797260). {ECO:0000269|PubMed:11882685, ECO:0000269|PubMed:1698710, ECO:0000269|PubMed:2475111, ECO:0000269|PubMed:2475570, ECO:0000269|PubMed:2606909, ECO:0000269|PubMed:36797260, ECO:0000269|PubMed:9053451}. The soluble form from urine retains its specific complement binding activity, but exhibits greatly reduced ability to inhibit complement membrane attack complex (MAC) assembly on cell membranes. . This is the function of CD59 (CD59 molecule (CD59 blood group), Ensembl gene identifier ENSG00000085063).