[Immunological recognition in pregnancy: physiology].

In the last decade works on the responses of the mother's immune system to the presence of the feto-placental unit have burgeoned. The antigens that are presented to the mother are special because the trophoblast does not show classical transplantation antigens which are classically implicated in graft rejection. A class I antigen, antigen HLA-G is present in the extra-villous cytotrophoblast, different from classical antigens by virtue of its molecular weight and its reduced antigenic polymorphism. Many functions have been attributed to this antigen; most of them are still being evaluated. The most important immunological phenomena are found at the feto-placental interface. Several events occur simultaneously or successively. Recognition of the trophoblast brings about an inflammatory reaction which is the initial phase of graft rejection. The numerous cytokines that are produced in this initial phase allow decidualization to occur and for the embryo to implant when it has reached an adequate stage of evolution. Rapidly, immunosuppressant mechanisms stop this rejection reaction which if not stopped can cause the pregnancy to end. There is a delicate equilibrium between the different cytokines, those favourable to pregnancy and those damaging to pregnancy. The trophoblast which is resistant to factors which would cause rejection protects the fetus particularly if its growth is helped along by certain cytokines. On the other hand, other cytokines are prejudicial to the growth of the trophoblast and activate certain cytotoxic cells which become aggressive. The maternal immune system and the endocrine system work together to maintain this cytokine network which if destabilized leads to certain pathological situations. Disturbances can be due to poor maternal recognition particularly if the trophoblast does not give out good antigens, or if the mother is genetically programmed not to respond although the disturbance can come from external factors such as certain infections.