Immunology in pregnancy is not well elucidated, although after intensive studies on the same. In pregnancy, trophoblastic tissue is considered as the foreign tissue and decidua is a part of the maternal tissue. Placenta is an important organ which has a variety of MHC antigens also called HLA -G antigens which causes immunosuppression and promotes cytokines for the growth and proliferation of the trophoblastic cells. These trophoblastic cells produce special signals to recruit the NK cells also called as decidual NK cells which lack the cytotoxic activity unlike the NK present in the peripheral blood. These decidual NK cells produce cytokines and growth factors for the placental growth and maturation. Trophoblasts produce progesterone which helps in inhibiting cytotoxic NK cells; it also induces the apoptosis of the same. Thus trophoblasts help in immunotropic function and blocks the immunoresponse locally. Immunoresponse begins in early pregnancy between mother and embryo .In pregnancy; there is stimulation of the TH2 cells and suppression of TH1 cytotoxic NK cells. if there is an imbalance between the these TH2 cells and TH1 cells rejection of the embryo occurs .Thus resulting in early pregnancy failure due to the activation of cytotoxic TH cells.

Keywords : immune system, implantation, trophoblast, major histocompatibility