Autoimmune activation toward embryo implantation is rare in immune-privileged human endometrium

Human embryo implantation represents embryo apposition, adhesion to the endometrial epithelium, and invasion into the stromal extracellular matrix within 1 to 2 days during days 6 to 9 after ovulation. The major molecular mechanisms mediating implantation include adhesion molecules, including mucins, selectins, integrins, and cadherins; extracellular matrix components, such as laminins and collagens and their degrading enzymes; phospholipids and immune regulatory molecules, including prostaglandins, cytokines; and immunosuppressive molecules expressed by invasive trophoblasts and endometrial cells. Many of these molecules are the targets for autoimmune reactions in autoimmune diseases and cancer; however, the relevance of those in immune-mediated implantation failure has not been defined. In this review, we will describe the molecules involved in 2-day event of human embryo implantation, which may also be involved in immune system activation and subsequently cause immune-mediated implantation failure. We speculate that the data in the literature are limited concerning antiendometrial antibodies because the endometrium might be taken as an immune-privileged site that avoids autoimmune activation that might harm the implantation process. Antibodies affecting human fertility in ways other than impairing implantation are outside the scope of the current article and will not be discussed.