Mechanisms of action of current and novel approaches to prevent and treat GVHD. APC, antigen-presenting cell; ATG, antithymocyte globulin; CSA, cyclosporine; CTLA4, cytotoxic T lymphocyte antigen 4; ECP, extracorporeal photopheresis; IL, interleukin; IL-6R, interleukin-6 receptor; ITAM, immunoreceptor tyrosine-based activation motif; JAK, janus kinase; LPS, lipopolysaccharides; MHC II, major histocompatibility complex II; MMF, mycophenolate; MSC, mesenchymal stem cell; mTOR, mammalian target of rapamycin complex; MTX, methotrexate; NFAT, nuclear factor of activated T cell; STAT, signal transducer and activator of transcription protein; Tac, tacrolimus; TCR, T-cell receptor; TNFα, tumor necrosis factor-α; Treg, regulatory T cell.