NELFCD and immunoglobulin G4-related sclerosing disease: Circulating Th1 and Tfh1 levels positively correlate with serum IgG4 levels and disease activity in patients with IgG4-RD, which might play an important role in the course of IgG4-RD, and could be the potential biomarkers for disease activity monitoring in IgG4-RD.