Survival of the genetically incompatible fetal allograft.

A hypothesis is put forward to account for the non-rejection of the fetal allograft, based on shared surface-repellent molecules (S.R.M.s) which prevent close apposition of maternal and fetal immunocompetent cells. This hypothesis is shown to be compatible with the major theoretical requirements of the immune system. Thus it provides possible explanations for the self/not-self recognition process, tolerance to organ transplants, the development of autoimmune disease, and high and low zone tolerance. It is also argued that the recognition of self is important with reference to cellular antigens, but may not be essential for humoral antigens.