Abstract: The definition of recurrent implantation failure (RIF) usually refers to failure to achieve a clinical pregnancy after going through at least three fresh/frozen cycles or transfer of at least four good-quality embryos in a woman under the age of 40 years. Embryo implantation is a complex process of the recognition and fusion between mater and fetus. On one hand, embryonic trophoblasts can express embryonic antigen and secrete cytokines in order to escape the attack of maternal immune system. On the other hand, the specialized immune cells of maternal decidua are enriched to develop a unique immune microenvironment at maternal-fetal interface. Both of them are beneficial for embryo implantation. Any immunologic disorder of the maternal-fetal interface during implantation may lead to implantation failure. With the development of reproductive immunology, the role of immune factors in the establishment of endometrial receptivity has been paid more and more attention, and the use of immunomodulators to improve the pregnancy outcome of RIF has been reported. In this paper, we described the recent research progress in immunological etiology of RIF and the related therapy.

Key words: Recurrent implantation failure, Embryo implantation, Immune tolerance, Cytokines, Therapy