CD4 and immunoglobulin G4-related sclerosing disease: Besides investigation of T helper (Th) cells and regulatory T (Treg) cells in the early period of IgG4-RD research, newly identified CD4+ T cell subsets, including T follicular helper (Tfh) cells, T follicular regulatory (Tfr) cells, CD4+ cytotoxic T lymphocytes (CD4+ CTLs), and T peripheral helper (Tph) cells, have been thoroughly explored.