ABSTRACT
In 1953, Medawar offered ideas for mechanisms that could account for the surprising ability of the fetal semiallograft to survive in a potentially hostile environment, i.e. anatomic separation of the mother and fetus, antigenic immaturity of the fetus, and tolerance in the mother (Medawar, 1953). All of these have now been identified, although they are somewhat different from Medawar’s original ideas. The blood circulations of the mother and fetus do not merge, cell surface molecules that stimulate graft rejection are late appearing in the fetus, and there is every evidence for the development of multiple tolerogenic mechanisms in the mother.
