Implantation site in normal pregnancy. A study with monoclonal antibodies.

In the present study, the presence of major histocompatibility complex antigens (MHC) and the degree and nature of inflammatory response in the human placenta were determined by staining frozen tissue sections with monoclonal antibodies and an immunoperoxidase technique. Although class I (HLA-A, B, and C) and Class II (HLA-DR, Ia-like) MHC antigens were not demonstrated in the syncytiotrophoblast, Class I antigens were found in trophoblast of the placental septum, shell, and implantation site and in the chorionic villous stroma. There was no staining for Ia-like antigens in the fetal components of the placenta. T cells were scarce and evenly scattered in the normal implantation site. No T cells infiltrated the chorionic villi. B cells and natural killer cells were not identified in the human placenta. Macrophages constituted more than 20% of the decidual cells and had morphologic features identical to those of "small decidual cells." The lack of T-cell infiltration of the fetal placental structures and their scarcity in the implantation site support the notion that T-cell-mediated immune response against placental antigens is not generated by the maternal host in normal pregnancy. The abundance of macrophages at the implantation site may be related to their possible role in the suppression of immune response.