Pathways of antigen presentation after allogeneic stem cell transplantation. Host alloantigens can be presented by host APC (direct presentation) or crosspresented by donor APC after uptake of particulate host material (indirect presentation). In direct presentation, donor T cells can potentially recognize allogeneic MHC antigens on host APC (MHC shown in red with or without a requirement for peptide) or may recognize host alloantigens (including allogeneic MHC or minor H antigens) that have been processed and presented by host APC. Alternatively, host allogeneic MHC or minor H antigens may be presented indirectly by donor APC.