In summary, the pathogenesis of IgG4-RD and MPA/GPA are complex and multifaceted, involving B cells, T cells, macrophages, and numerous cytokines (e.g., IL-4, IL-13, IL-21, IL-17, TGF-β) (Table 1). The gene discussed is IL21; the disease is immunoglobulin G4-related sclerosing disease.