CD4 and immunoglobulin G4-related sclerosing disease: In addition to the crucial roles of B cell subpopulations as antigen-presenting and effector cells in the pathogenesis of tissue fibrosis in patients with IgG4-RD, recent investigations have further revealed that many innate immune cells including white blood cell lineages (neutrophils, basophils and eosinophils), and macrophage type 2 (M2–Mφ)/plasmacytoid dendritic cells (pDC) [117,118] and different T cell subsets including, Th2, Treg, Tfh2 and CD4+ - and CD8+-cytotoxic T cells (CD4+and CD8+ Tc) play active and critical roles in the immunopathogenesis of IgG4-RD [101].