The histocompatibility system is responsible for the rejection of allograft. The system exists to counter the explosive speed of viral replication. It does this by directing the defensive immune attack by cytotoxic T cells on to histocompatibility antigens on the infected cell’s surface. This enables destruction of the virus factories that the infected cells become, before the cytotoxic T cells are swamped by the myriad numbers of new virions, a thousand coming from each infected cell every 10 hours. The immunity system mistakes alloantigens for virus infected host cells that need swift destruction. For transplantation, Sykes has improved Kaplan’s technique by adding recipient bone marrow cells to the donor ones injected for reconstitution of the recipient after immune ablation. This protocol should enable xenografting from untreated pigs, offering instant and unlimited supply of grafts for man.