Recipient treatment to overcome the allograft reaction, with special reference to nature's own solution.

The 6th decade of this century was particularly important for transplantation immunology. The universality of allograft rejection by normal hosts had won general acceptance and, experimentally, several means of abrogating host reactivity to allografts were discovered. These included sub-lethal whole body irradiation, administration of certain corticosteroid hormones and inoculation of very young animals with living cellular inocula from the future graft donor--i.e., classic, neonatal tolerance. The latter was particularly important since it indicated the feasibility of a specific, permanent solution to the clinical allograft problem. Radiation and drug-induced tolerance in adult subjects came along and chemical immunosuppressants, which led to successful clinical use of azathioprine. The important rediscovery of ALS pointed towards the development and clinical application of monoclonal antibodies many years later. With the development of immunogenetics and transplantation biology came recognition that the conceptus is a highly successful allograft, raising the question of how it is able to withstand rejection by its immunocompetent mother for the duration of pregnancy. Hopefully, knowledge of the principle(s) involved when they are finally elucidated will be applicable to clinical allograft recipients. Although functional hypoantigenicity of the syncytial trophoblast probably plays a major role in protecting the allogeneic conceptus, a strong case now exists that local, cell-based, immunosuppressive and immunoprotective activity within the placenta and decidua, mediated by suppressor and other cells, is important.