Donor T-helper cells (Th) play a crucial role in the triggering of GVHD due to their capacity to distinguish 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, contributing to organ-specific GVHD [117]. The gene discussed is IL17A; the disease is graft versus host disease.