Placenta as an Immunological Barrier

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Abstract
The immunoregulatory mechanisms operating during an allogeneic [vertebrate or human] pregnancy that contribute to the successful survival of the fetoplacental unit as an allograft are highly complex. There is no simple, singular immunological explanation for the establishment of immune coexistence between a mother and her fetus during gestation. Immunological processes and/or factors apparently are unequivocally initiated during gestation, although there is still some question as to the fetal or the placental source of the antigenic stimulus. At present, the normal immunologic reactivity of the female to alloantigens of the fetus has not been shown to bring about cytotoxic destruction of trophoblastic target cells in vivo or in vitro. However, the fact that the cytotrophoblast of the anchoring villi express HLA-A, B, and C antigenic determinants prompts relooking at this frontier area in well-defined reproductive disorders such as toxemia of pregnancy, intrauterine growth retardation or abruptio placentae. The alterations in immunoreactivity coupled with the production of unique protein and steroid hormones by the trophoblast probably result in an effective blockade of the efferent limb of the immunologic reflex arc as far as effector lymphocytes are concerned and allow the trophoblastic epithelium to experience an immunologically privileged status as a tissue. At the present time no unifying hypothesis or hypotheses exist that assemble all the known basic facts regarding the immunological response machinery which give a plausible explanation for the inordinate complexity of this system. This system under most circumstances seems foolproof without major cost to the mother and certainly is of major benefit to the fetus and placenta, preserving the individuality of both.