This review discusses the role of CD4+ T cell subsets in the pathogenesis of IgG4-RD described in recent reports and shows the results of our recent study on the interaction between Tph cells and extrafollicular B cells in IgG4-RD as a new topic of IgG4-RD pathophysiology. The gene discussed is TPH1; the disease is immunoglobulin G4-related sclerosing disease.