C5 and rheumatoid arthritis: Intriguingly, a nonsynonymous coding SNP (rs1926447; T325I) in the gene for carboxypeptidase B (CPB), a plasma inhibitor of C5a activity, is associated with decreased risk of severe RA [67]; the ‘protective’ CPB variant displays increased plasma half-life and C5a-inhibiting activity, suggesting a mechanism for the observed protection.