Abstract
Half a kilogram of immune cells reside in tissues. In the uterus, innate lymphoid cells (ILC) contribute to the cyclic destruction and repair of the mucosa. During pregnancy, uterine ILC support the formation of the placenta and the growth of the fetus. They also contribute to immune responses to pathogens. ILC respond quickly to signals of tissue perturbations and, by influencing other immune cells, they organise responses that help maintain tissue health. Their functions have been determined in the respiratory and intestinal tracts, skin, liver and adipose tissue. It is challenging to determine the function of uterine ILC because of the cyclic changes of the endometrium and the difficulties in accessing human tissues during pregnancy. We review the existing literature on the involvement of uterine ILC in physiology and pathology of the non-pregnant endometrium as well as in pregnancy, from implantation of the fertilised egg to the tissue remodelling occurring during the first trimester and that leads to the formation of the placenta which sustains fetal growth, until parturition.