Donor T-helper (Th) cells are crucial in GvHD initiation, owing to their ability to differentiate into Th1 (secreting IL-2 and IFN-γ), Th2 (secreting IL-4, IL-5, IL-10 and IL-13), Th17 (secreting IL-17), and Tfh cells, which facilitate organ-specific GvHD (44). This evidence concerns the gene IL5 and graft versus host disease.