Table 3.
Type of placenta is species specific and determines route of transfer of maternal antibodies.
| Species | Placenta | Maternal antibody transfer | Transfer mediated by neonatal Fc receptor (FcRn) | Reference |
|---|---|---|---|---|
| Human | Hemochorial | Transplacental | Yes; preferential transport of IgG1 > IgG3 > IgG4 > IgG2 | (91–94) |
| Rodents | Hemochorial | Transplacental/colostrum | Yes | (6, 95–98). |
| Mouse | ||||
| Rat | ||||
| Cotton rat | Hemochorial | Transplacental/colostrum | Unknown | (99, 100) |
| Dogs and cats | Endotheliochorial | Low transplacental/high in colostrum | Unknown | (12, 20, 40, 101–104) |
| Cattle, sheep, pigs, and horses | Epitheliochorial | Colostrum | FcRn present in pig intestine, role in IgG transfer questionable | (94, 105–107) |
| Birds | None | In ovo | FcRY (bird equivalent to FcRn) | (108, 109) |
The three potential maternal layers in a placenta are the endothelium lining of endometrial blood vessels, connective tissue of the endometrium, and endometrial epithelial cells. In humans and rodents, the fetal chorionic epithelium is in direct contact with maternal blood because only the maternal endothelium remains (hemochorial placenta). In contrast, the chorionic epithelium of horse and pig fetuses remains separated from maternal blood by three layers of tissue (epitheliochorial placenta) whereas only two layers remain in dogs and cats (endotheliochorial placenta). The fewer the layers between maternal and fetal blood the higher the rate of transport of maternal antibodies transplacentally. In humans, the majority of maternal IgG is transferred via the placenta. The transfer is an active process during which IgG binds to the neonatal Fc receptor (FcRn), which is located in the placental syncytiotrophoblast. The binding between FcRn and IgG is 100-fold higher at pH 6 than at pH 7.4 (91, 92). After endocytosis of IgG by the placental syncytiotrophoblast IgG binds to FcRn in the endosome at low pH, is transported to the opposite cell membrane and released at physiological pH into the blood stream [reviewed by Palmeira et al. (110)]. This mechanism explains why maternal antibodies are of the IgG isotype. In contrast to this specific transport mechanism, it seems that in agricultural species IgG and other macromolecules are transported from colostrum across the intestinal barrier in a non-specific fashion (111, 112). Within 24-h after birth, the intestine becomes impermeable to macromolecules and IgG transfer ceases. Similar to rodents, e.g., pig intestinal epithelial cells also express FcRn during the neonatal period but it has been argued that the large amount of IgG taken up within 24 h is not consistent with receptor-mediated transport [discussed in Ref. (107)]. In contrast to rodents, pig FcRN also is expressed in the adult period of life (106) and this expression pattern is not correlated with transfer of maternal antibody. In contrast to mammals, birds generate IgY antibodies instead of IgG antibodies and use the IgY Fc-receptor (FcRY) to transport IgY into the egg.