The p→F1 model using B6D2 F1 mice thus allows for the comparison of in vivo immune responses in the setting of either a strong Th1-dominated response driven by high IL-2 production (B6 GVHD), or a Tfh-dominated response in the presence of low IL-2 levels (D2 GVHD). This evidence concerns the gene IL2 and graft versus host disease.