Natural Killer (NK) Cells

By RRM Academy Reviewed by Dr. Naomi Whittaker, MD, Board-Certified OBGYN, MIGS, NFPMC, FCI Updated May 7, 2026

Natural killer cells are a type of innate immune cell found throughout the body. In the uterine lining, a specialized population called uterine natural killer cells (uNK cells) plays a distinct role during implantation. These cells help remodel the spiral arteries of the uterus and regulate the immune environment at the maternal-fetal interface. They are not simply immune attackers. They are part of the architecture of a successful implantation.

The relationship between NK cell activity and fertility is not straightforward. In endometriosis, altered NK cell function has been documented: women with endometriosis show reduced cytotoxic activity against autologous endometrial cells, which may allow ectopic tissue to survive and proliferate.⁶¹ This immune dysregulation is one mechanism by which endometriosis may compromise implantation and contribute to both infertility and recurrent loss.

Peripheral blood NK cell testing measures circulating NK cells, not uterine NK cells. The two populations behave differently. Elevated peripheral NK cell counts do not reliably predict uterine NK cell dysfunction. Clinicians who draw conclusions about implantation failure from blood NK counts alone are measuring the wrong compartment.

RRM clinicians who evaluate implantation failure look at the uterine environment directly, through endometrial biopsy, progesterone levels, and cycle timing, rather than relying on peripheral markers that have uncertain clinical correlation. Treating elevated NK cell counts on a blood test without diagnosing the underlying cause is not restorative medicine.

When genuine immune dysregulation is contributing to implantation failure or recurrent loss, addressing the root condition matters. In endometriosis-related cases, excision of disease followed by targeted luteal phase support is the restorative approach. The goal is a uterine environment capable of sustaining pregnancy, not immune suppression layered on top of an unaddressed diagnosis.

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.