The serum IL-21 level. The serum IL-21 levels of patients with primary SS (n = 40), SLE (n = 38), or RA (n = 40) and healthy controls (n = 20) were measured by ELISA. The primary SS patients had significantly higher serum IL-21 levels than the healthy controls or the RA patients. Horizontal bars represent the median of all samples within a group. * P < 0.05, ** P < 0.01. IL: Interleukin; RA: rheumatoid arthritis; SLE: systemic lupus erythematosus; SS: Sjögren's syndrome.