C4A and immunoglobulin G4-related sclerosing disease: Our study found that compared with IgG4-RD-non-RPF, the levels of eosinophils, IgG4 and IgG4/IgG were significantly decreased, and the levels of neutrophils, C3 and C4 were increased in IRPF patients, which indicated that the pathogenesis of the two diseases was different.