Establishment of pregnancy and its immunological implications in the pig

The consideration of the fetus as a semi-allograft by virtue of its paternally inherited histocompatibility antigens is generally accepted. Many hypotheses have been put forward to explain the immunological acceptance of the conceptus by the maternal immune system, but no single theory has proved to be adequate. Most studies so far have been carried out with laboratory rodents and men but corresponding investigations of farm animals and especially the pig are limited. Because striking differences exist between various species, e.g. with respect to the type of placentation, litter size, gestation time and hormone production, the relevance of studies in rodents and men for other species has to be established. This review examines, in the context of available information from other mammalian systems, whether the pig conceptus expresses and presents antigens to the maternal immune system, to what extent the sow recognizes and responds to these antigens, and, in the event of recognition, the mechanisms by which the conceptus avoids and exploits the consequences of histoincompatibility.