Positive correlation of high serum IL-21 levels with disease activity and auto-antibody production in patients with rheumatoid arthritis (RA). (a) Serum IL-21 levels of RA patients (n = 120) and healthy controls (HC) (n = 80) were measured by ELISA. (b) There was positive correlation between serum IL-21 and 28-joint count disease activity score (DAS28) in RA patients with high IL-21 (n = 38). (c) There was a positive correlation between serum IL-21 and levels of anti-cyclic citrullinated peptide (anti-CCP) antibody in RA patients with high IL-21 (n = 38). (d) Serum IL-21 had no relationship with levels of rheumatoid factor (RF)-IgM, RF-IgG or RF-IgA in RA patients with high IL-21 (n = 38). (e) Serum IL-21 levels in RA patients treated with different drugs. **P < 0.01; *P < 0.05. DMARDS, disease-modifying antirheumatic drugs; CS, corticosteroids.