Step 1: priming of the immune response. Conditioning regimens used to prepare recipients for allogeneic hematopoietic stem cell transplantation (HSCT) cause graft-versus-host disease (GVHD) parenchymal organ injury and the release of proinflammatory cytokines that initiates allogeneic priming. The red boxes below the mouse and the human recipient serve to highlight distinct features between these species. A major unresolved issue not explained by this schema is shown in the middle red box. RIC denotes reduced-intensity conditioning.