Donor Th1 cells preferentially secrete type-1 cytokines (IL-2, IFN-γ, and TNF-α) and induce GVHD, whereas donor Th2 cells, which secrete type-2 cytokines (IL-4, IL-5, IL-10, and IL-13), could reduce GVHD (39–41). Here, IL4 is linked to graft versus host disease.