C3 and systemic lupus erythematosus: Serum levels of antinuclear IgG4 in SLE patients were positively correlated with serum levels of total IgG4, albumin, and C3 (r = 0.61, P < 0.05; r = 0.40, P < 0.05; and r = 0.54, P < 0.05, resp).