Abstract
Natural killer (NK) cells are lymphocytes involved in the innate immune response against certain microbial and parasitic infections (1, 2). These cells comprise ≈5–20% of peripheral blood lymphocytes and are present in spleen, liver, bone marrow, and thymus at a lower frequency and are rare in lymphatics and lymph nodes. However, in the mucosal decidual tissues of the maternal uterus, NK cells are the most abundant class of lymphocyte, representing up to 95% of all lymphocytes. Initially identified by their distinct morphology, NK cells in the placenta were designated “granulated metrial gland cells” in rodents (3) and “uterine granular lymphocytes” in humans (4, 5). Why do NK cells preferentially accumulate in the decidua? No clear rationale has emerged to explain this phenomenon. Given the established role of NK cells in innate immunity, protection of the fetus against transplacental infections is a good bet, but nonimmune functions involving fetal sustenance have also been proposed (6).
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