The secretory immune system as part of the placental barrier in the second trimester of pregnancy in humans.

The objective of this study was to describe the development of the secretory immune system (SIS) in the placenta of 32 human fetuses who had died from different causes during the second trimester of pregnancy. The distribution of SIS protein elements, including the secretory component (SC), J-chain, IgA, IgM, IgG, as well subsets of lymphocytes and macrophages, were studied by immunohistochemical methods. Both the fetal and maternal parts of the placenta were found to contain these elements. In the fetal part, the immunoglobulins, SC and J chain were located in the syncytio- and cytotrophoblast of the chorion and in the epithelium of the amnion. The villous stroma contained a small amount of different subsets of lymphocytes. Macrophages accounted for up to 45% of the stromal cells of the villi and contained IgG and J-chain. In the maternal part of the placenta, the SIS proteins were in the decidual cells. Relatively few lymphocytes and macrophages were observed in the decidual stroma. Our data suggest that the placenta has two different SIS, one in the fetal part and the other in the maternal part. They differ in their structure and orientation: the maternal SIS protects the mother against paternal antigens from the fetus, while the fetal SIS protects the fetus against macromolecules and infectious agents penetrating from the mother. The placenta represents the largest extracorporal immune system that is functionally active during the whole second trimester of gestation. We suggest that the concept of placental barrier should be expanded to include both fetal and maternal parts of the SIS, fetal membranes and the decidual tissue.