A review of immune transfer by the placenta.

Feto-maternal immune transfer occurs via both the placenta in utero and colostrum after birth. The layers between the maternal and fetal circulation systems, known as the placental barrier, regulate immune transfer to the fetus via the placenta. The placental barrier, as well as the type of placental structure, is species specific. The extent of transfer of antibodies from mother to fetus is related to the number of placental barrier layers. Passive immunity via the colostrum is essential in species in which the type of placentation impedes contact between maternal and fetal circulation systems, hindering the transfer of antibodies. In these species, susceptibility to neonatal infections is increased if colostrum is not ingested. Acquired antibodies are of extreme importance for adaptation of the neonate to the extrauterine environment. Based on the aforementioned factors, it was observed that in synepitheliochorial and epitheliochorial placentas immune transfer via the placenta is not possible, except in cases of placental alteration (e.g., placentitis). On the other hand, the mechanism of transfer in endothelial and hemochorial placentas is facilitated compared with other placentas. We conclude that there are no appreciable qualitative differences between the two mechanisms of transfer (placenta and colostrum) and that immune protection in the neonate can be attained by either mechanism.