The early pregnant human endometrium (decidua) contains a diverse population of leukocytes that mediate innate immunity, including uterine natural killer (uNK) cells (7075%) and macrophages (20-25%). In contrast, T and B lymphocytes, which mediate adaptive immunity, account for 10% and between 1% and 3 % of the white cell population, respectively. In recent years, research has focused on the role played by uNK in placentation, with far less attention being directed towards decidual macrophages.
Nevertheless, several crucial pregnancy-related functions have been ascribed to these leukocytes, ranging from the utero-placental response to infection and the production of cytokines, to the establishment of maternal tolerance and tissue remodeling during implantation and trophoblast invasion. An excess of decidual macrophages has also been implicated in the impaired endovascular trophoblast invasion that underlies pre-eclampsia and intrauterine growth restriction.
Histological studies have revealed that the decidual leukocyte cell population undergoes dynamic changes throughout gestation. At the implantation site, uNK cells increase in number during the first trimester, and then gradually decline until they are virtually absent at term.