IMMUNOLOGY OF PREGNANCY
Although the maternal immune system is not altered in pregnancy, the antigenically dissimilar fetus is able to survive in the uterus without being rejected. The key to this successful fetal allograft appears to be the placenta. The placenta serves as an effective interface between the maternal and fetal vascular compartments by keeping the fetus from direct contact with the maternal immune system. The placenta also produces estrogen, progesterone, hCG, and hPL, all of which may contribute to suppression of maternal immune responses on a local level. In addition, the placenta is the site of origin for blocking antibodies and masking antibodies, which alter the immune response.
The mother’s systemic immune system remains intact, as evidenced by leukocyte count, B and T cell count and function, and immunoglobulin (Ig) levels. Because IgG isthe only immunoglobulin that can cross the placenta, maternal IgG comprises a major proportion of fetal immunoglobulin, both in utero and in the early neonatal period. In this fashion, passive immunity is transferred to the fetus.
In this environment, the fetal immune system is afforded the opportunity to gradually develop and mature by term. Fetal lymphocyte production begins as early as 6 weeks of gestation. By 12 weeks of gestation, IgG, IgM, IgD, and IgE are present and are produced in progres-sively increasing amounts throughout pregnancy. At birth, the newborn fetus is equipped with both passive immunity and a mature immunologic system to defend against infec-tious diseases