[Immunology of pregnancy]

One of the phenomena that offers more questions in Immunology is: Why the embryo, behaving like a semialogenic graft, is not rejected by the mother? It is known that the mother produces an active immunological response against the fetus and, nevertheless, in normal conditions, the immunological rejection does not take place. In the present work of revision, some mechanisms are described by means of which the specific immunological tolerance of the mother is generated towards the embryo. All these mechanisms are interdependent and altogether they constitute a safety network to avoid the fetal rejection. Here the effects of female sex hormones on the immunological system, the change of the profile of cytokines, the generation of immunomodulating proteins and blocking antibodies, the effect of the expression of the HLA-G and the paper of some cells, like Regulatory T lymphocytes, dendritic and Natural Killer cells, are detailed. Also other routes by which the embryo defends itself of the maternal immunological attack are described, like the induction of apoptosis in the endometrium and leukocytes, the tryptophan and iron metabolisms, the inhibition of the complement system and the expression of annexins.