**3. Localization of NK cells**

NK cells are present in the bloodstream and in the lymphatic vessels [12], as well as in placenta, spleen, liver, lungs, tonsils, peripheral ganglia, and bone marrow where they act as sentinels. Nevertheless, it appears that they have no access to other tissues apart from inflammatory responses. Of note, it has been reported that the homeostasis-driven NK cells can reside in both lymphoid and nonlymphoid organs for a long time [13].
