Allogeneic CAR T cells: balancing antitumor vs graft host disease. BM donor CAR T cells directed to CD19 (CAR19) recognize CD19 on B-ALL cells and release effector proteins (perforin and granzyme B) inducing B-ALL apoptosis. Cytokines (in particular IL-6) can be released at high levels, inducing CRS, a serious clinical toxicity. In addition, a subset of donor CAR19 cells (mostly CD4⁺ CAR19) recognize host alloantigen and induce GVHD. Data from Jacoby et al, and others, indicate both GVHD and CRS can be alleviated by careful timing of the donor CAR19 infusion and using the IL-6R antibody (tocilizimab). APC, antigen-presenting cell; IFNγ, interferon γ; TCR, T-cell receptor. Professional illustration by Ken Probst, XavierStudio.