Phantom Tc17 cells mediate GVHD but not GVL. Under the influence of IL-6, CD8⁺ T cells differentiate into weakly cytolytic lymphocytes that secrete IFN-γ but only transiently secrete IL-17 (Tc17). These cells cause GVHD but do not mediate GVL effects. When IL-6 is absent, CD8⁺ T cells differentiate into potent cytolytic T lymphocytes that mediate GVL but do not cause GVHD. Professional illustration by Patrick Lane, ScEYEnce Studios.