A total of five complement proteins (A2M, CFB, CFD, CFHR5, and F9) and two cytokines (IL1RN and TNFRSF17) were differentially abundant specifically in ACPA– RA, whereas only one complement protein (C9) was differentially abundant specifically in ACPA+ RA (Figure 3D). Here, A2M is linked to rheumatoid arthritis.