TNFRSF13C and immunoglobulin G4-related sclerosing disease: As IgG4-RD patients are characterized with hypergammaglobulinemia and high serum IgG4 levels, we hypothesize that B cells from IgG4-RD patients might be in an endogenously activated state, with highly expressed BAFF-R, and readily differentiate into IgG/IgG4-secreting plasma cells.