Anti-CCP2 IgG contains autoantibodies directed to citrullinated proteins/peptides (ACPAs) of various fine specificities. Analysis was conducted by enzyme-linked immunosorbent assay (ELISA). ACPA responses in purified anti-CCP2 IgG eluates were measured by ELISA in 10 plasma samples (left panel) and 11 synovial fluid samples (right panel). Anti-CCP2 IgG response (top panel), anti-CEP-1 IgG response, anti-Cit-vim IgG response, anti-Cit-fib IgG response, and anti-Cit-C1 IgG response (bottom panel) are shown. X-axis shows anti-CCP2 IgG eluate concentrations (μg/mL). Y-axis shows ACPA levels (AU/mL). Anti-CCP2 IgG eluates highlighted in red indicate samples with particularly weak binding to the CCP2 ELISA plates. CCP, cyclic citrullinated peptide; CEP-1, citrullinated α-enolase peptide 1 (amino acid 5-21); Cit-C1, citrullinated triple helical peptide/epitope on collagen type II (amino acid 359-369); Cit-fib, citrullinated fibrinogen peptide (β-chain, amino acid 36-52); Cit-vim, citrullinated vimentin peptide (amino acid 60-75); IgG, immunoglobulin G.