White blood cells or lymphocytes that form part of the 'innate' immune system capable of recognising foreign cells and dissolving them (hence their name: natural killer cells). Found normally, however, in the endometrium of the late secretory phase and in the decidua of pregnancy, where they have more friendly purposes in forming the boundary between mother and fetus. Unlike T cells and B cells, most NK cells do not have 'immunological memory', whereby previous contact with a particular antigen causes a faster, more deadly immune response second time round.

The outermost layer of trophoblast, produced by cytotrophoblast where there is direct contact with maternal decidua rather than blood. The EVT cells travel into the decidua, reacting with NK cells and invading maternal blood vessels feeding the placenta, softening the walls and replacing the lining with fetal tissue, a process called "conversion". Also known as interstitial or intermediate trophoblast.