ABSTRACT
Preimplantation embryos express paternal as well as maternally derived proteins, yet they are not rejected by the maternal immune system. This enigma was first recognized by Medawar over 50 years ago1 when he wrote ‘. . . how does the pregnant mother contrive to nourish within itself, for many weeks or months, the foetus that is an antigenically foreign body? Is pregnancy accompanied by any physiological changes which may in some degree prevent the foetus, qua tissue homograph, from immunizing the mother against itself?’ In spite of extensive research in this area, our knowledge of how this comes about is still incomplete.
