Feto-maternal bidirectional mixed lymphocyte reaction and survival of fetal allograft.

Maternal and fetal lymphocytes were tolerant of each other in the bidirectional mixed lymphocyte reaction. This seems to be the principal reason why the mother does not reject the fetal allograft. Tolerance between maternal and fetal cells must be largely due to a genetic mechanism, because the bidirectional mixed lymphocyte reaction between parents and older children was much reduced compared with that between randomly selected pairs of controls. This weak reaction disappeared when immunosuppressive agents were given to one member of the parent/child pair, whereas the mixed lymphocyte reaction between unrelated individuals was not abolished by similar immunosuppression. It is suggested that this genetic mechanism is distinct from the HLA system. Tolerance between maternal and fetal cells was not demonstrated in the unidirectional mixed lymphocyte reaction, suggesting that this tolerance requires the viability of the two cell populations.