Correlations between the serum IL-21 levels of primary SS patients and the levels of immunoglobulin. (a) The serum IL-21 levels of primary SS patients (n = 40) correlated significantly with the serum globulin levels. (b) Of all the Ig classes, the serum IL-21 levels only correlated with IgG (n = 40). (c) There was a positive correlation between the serum IL-21 and IgG1 levels (n = 24). (d) The anti-Ro/SSA antibody titers, as measured by ELISA, correlated with the IgG1 levels (n = 35). (e) The serum IL-21 levels correlated with anti-Ro/SSA antibody index (n = 24) and IL-21 was an independent variable in regression analysis. The correlation coefficients (r) and P-alues of statistical significance are shown. IG: immunoglobulin; IL: Interleukin; SS: Sjögren's syndrome.