CD4 and immunoglobulin G4-related sclerosing disease: Recent immunohistochemical studies showed that CD4+ T cells are the most abundant and active cells within the lymphoplasmacytic infiltrate in the affected organ in IgG4-RD patients, suggesting that, in addition to B cells, T cells could be key players in IgG4-RD pathogenesis [6, 19, 27, 33, 37–39].