Where are natural killer cells most commonly found during pregnancy?

Natural killer (NK) cells play a crucial role in pregnancy, particularly in the maternal-fetal immunological interface. They are most commonly found in the decidua, which is the modified endometrial lining of the uterus that forms during pregnancy. The decidua provides a unique environment where maternal immune cells, including NK cells, are involved in remodeling blood vessels and establishing the placental interface. These cells are thought to contribute to the regulation of trophoblast invasion and support the development of the placenta.

The presence of NK cells in the decidua differs from their distribution in other locations during pregnancy, as they are less abundant in the placenta itself, maternal blood, and amniotic fluid. While maternal blood contains circulating NK cells, their primary site of action and concentration during pregnancy is the decidua, where they are specifically adapted to facilitate a successful pregnancy outcome by balancing the immune response towards tolerance and protection of the growing fetus. This unique immunological niche allows them to perform critical functions in responding to the needs of the developing placenta and fetus.