CD4 and immunoglobulin G4-related sclerosing disease: Activated B cells present antigens to T cells via major histocompatibility complex class II, stimulating CD4+ T cells to differentiate into cytotoxic T lymphocytes (CD4+CTLs) and secrete chemokines such as C-C motif chemokine ligand (CCL)-5, which attract CD4+CTLs to affected tissues in IgG4-RD (37).