Granulated lymphocytes of pregnancy

In pregnant mammals, the antigenically distinctive conceptus implants and grows in a uterine environment governed by the maternal immune system. The uterus per se is not immunologically privileged and large numbers of lymphocytes accumulate at implantation sites. In mice and humans, many of these lymphocytes have been identified as uterine natural killer (uNK) cells and exhibit a characteristic granulated morphology. In this review we focus on uNK cells and discuss their origin, differentiation and possible roles in the maintenance of healthy pregnancies. In species with less invasive placentation (ruminants, pigs), lymphocytes with similar granular morphology also appear during gestation and their identity and possible functions are examined.