The immune function of the endometrium

The endometrial mucosa is unique amongst mucosal sites in that it must mount an immune response against micro-organisms and resist tumour growth whilst tolerating sperm and the allogeneic fetus. Bacterial and viral infection in the uterus leads to local endometrial mucosal immune responses evidenced by the secretion of secretory component (SC), secretory IgA (sIgA) and IgG. The secretion of these molecules is under hormonal control. Trafficking of locally sensitized lymphocytes to other mucosae does not appear to occur, whereas priming at other mucosal surfaces leads to memory responses to antigen in the uterus. Proclivity to local immune function is related to sparse lymphatic supply to the endometrium together with a local distribution of antigen-presenting dendritic cells. During pregnancy, particularly in the region of the decidua and embryo, the number of Ia+ cells and the lymphatic supply become diminished. The antigenic status of sperm may lead to certain types of maternal sensitization. However, immunosuppressive factors in seminal plasma protect the sperm on its passage up the female genital tract and diminish subsequent immunogenicity. On fertilization of the oocyte, an allogenic fetus develops, that potentially could stimulate production of maternal immune effectors. Endometrial cells of various types (macrophage, decidual, NK, T cells) interact via soluble factors leading to a local immunoprotection of the fetus. Similar factors appear to operate in resisting tumour growth in the uterus.