Natural Killer (NK) Cells
Natural killer (NK) cells are lymphocytes of the innate immune system with two populations relevant to reproductive medicine: circulating NK cells (CD56dim), which perform immune surveillance in the bloodstream, and uterine NK cells (CD56bright), which are the predominant immune cells in the endometrium during the secretory phase and early pregnancy.[209]
Uterine NK cells support normal placentation. They facilitate trophoblast invasion and drive spiral artery remodeling, both essential for adequate blood flow to the developing pregnancy. Without this remodeling, placentation is shallow and downstream pregnancy complications rise.[209]
Elevated uterine NK cell activity has been associated with implantation failure and recurrent pregnancy loss in a subset of patients, though clinical testing and optimal management remain areas of active investigation rather than settled protocol.[37] Chronic endometritis, an infection-related inflammatory state, may alter the local immune environment in ways that affect NK cell behavior.[26] The relationship between the endometrial microbiota, uterine NK activity, and implantation is an emerging area of study.[65]
RRM clinicians may factor uterine immune evaluation into the workup of unexplained implantation failure or recurrent pregnancy loss as part of a broader assessment.
This content is for educational purposes only and does not constitute medical advice. Consult an RRM clinician or healthcare provider for guidance specific to your situation.