Beyond the embryo: reframing recurrent implantation failure through immunopriming and endometrial immune dysregulation.

Recurrent implantation failure (RIF) remains one of the most debated and emotionally challenging conditions in assisted reproductive technology (ART). Despite remarkable advances in embryology, including blastocyst culture, time-lapse imaging and preimplantation genetic testing for aneuploidy (PGTA), a significant proportion of patients continue to fail implantation even after transfer of morphologically good-quality or euploid embryos. Contemporary reproductive medicine has largely adopted an embryocentric model of implantation, often defining RIF based upon repeated failed embryo transfers. However, this framework may represent a statistical oversimplification that inadequately reflects the biological complexity of implantation and the clinical realities faced by infertile couples. Emerging evidence increasingly supports the concept that implantation is not solely dependent on embryo competence but also requires synchronized endometrial receptivity and finely regulated maternal immune tolerance. The endometrium functions as an active immunological organ populated by uterine natural killer cells, macrophages, dendritic cells, cytokines, chemokines, and regulatory T cells, all of which contribute to embryo acceptance, trophoblast invasion, angiogenesis, and placentation. Dysregulation of this immune-endometrial dialogue may contribute substantially to implantation failure in selected patients. This perspective article critically examines the limitations of current RIF definitions, the overemphasis on embryo quality, and the under-recognized role of reproductive immunology. Furthermore, it explores the emerging concept of immunopriming and discusses potential immunological markers, peripheralendometrial immune crosstalk, individualized immune profiling, and immunomodulatory therapies such as intravenous immunoglobulin (IVIG) and lymphocyte immunotherapy (LIT). Although current evidence remains heterogeneous and definitive randomized controlled trials are lacking, outright dismissal of immune contributions to implantation failure may no longer be scientifically justified. We propose that implantation failure should be reconsidered as a multifactorial disorder involving embryo competence, endometrial receptivity, and immune equilibrium, thereby advocating for a more integrated and individualized approach to implantation medicine.