The maternal immune system does not ignore the fetal allograft. Antibodies specific for paternally inherited Rhesus D antigen and for HLA molecules — or, rarely, T cells specific for mismatched minor histocompatibility antigens — are found in the maternal circulation. However, these T cells do not normally reach the fetus itself, as this is protected by several mechanisms, including the placental barrier. The uterine mucosa is in direct contact with the fetal placenta at the maternal-fetal interface. This is the major site where fetal placental cells (not the embryo proper) are directly in contact with maternal tissues. The decidua is a specialized tissue that is rich in uNK and myeloid cells and also contains maternal T cells, including effector T cells and Tregs. Although it appears clear that uNK cells have receptors that can interact with HLA-C molecules on invasive trophoblast cells at the interface, how effector T cells might interact with the trophoblast cells is unclear. Circulating in the maternal blood are antifetal antibodies and T cells together with fetal cells and trophoblast cells.