According to most studies, conditioning regimens initiate aGVHD by triggering the production of cytokines (such as TNF-α, IFN-γ, IL-1, and IL-2) which, in turn, up-regulate the chemokine receptors and their ligands that drive the migration of T cells to GVHD target tissues (53, 55, 80, 81). This evidence concerns the gene IL2 and graft versus host disease.