That’s a scary term: Natural Killer Cell. But these white blood cells are actually very important to the immune system, serving as a first line of defense against viruses and even cancer. Their relationship with fertility is a more complicated one, however. Some types of Natural Killer Cell (NK cells) may help a fetus implant and develop, but elevated counts of other NK cells may be linked to failed implantation or even miscarriage.

What Are Natural Killer Cells?

White blood cells form the core of your body’s immune system. Most white blood cells react to markers called antigens that recognize known threats. NK cells are white blood cells that respond to infection without needing these markers, and therefore are often the first immune cells to respond to diseases. They release chemicals that kill off the infected cells without damaging healthy cells around them.

NK cells were first discovered because of their ability to recognize and attack tumors. Later they were shown to attack infected cells, but it took even more time to develop a theory of how they work. The exact mechanisms are still debated, but most researchers believe that NK cells work on a principle of “missing self.”

Nearly all healthy cells give off chemicals that identify them as normal parts of the body, or “self.” NK cells read these chemicals and pass healthy cells by. But unhealthy cells, like tumors and cells infected by disease, do not give off the correct chemicals — so NK cells attack. In this way, NK cells don’t have to wait for other immune cells to take a closer look and learn what the danger is.

NKCs and Fertility

NK cells are found throughout the body, circulating in the blood stream. However, during pregnancy, NK cells are found in high numbers in the uterus. These uterine NK cells seem to work differently than those in the blood, which are also known as peripheral NK cells. Exactly how uterine NK cells work isn’t fully understood, but it is believed they help the uterine lining and the placenta work together to transfer blood and nutrients to the developing fetus.

Unfortunately, peripheral NK cells may prevent an embryo from successfully implanting or proceeding to term. While some studies have connected high levels of peripheral NK cells to recurrent implantation failure, overall the evidence is inconclusive. Elevated levels of peripheral NK cells are often found in women who experience recurrent miscarriage, however.

Some theories link inflammation from diseases like PID and endometriosis to peripheral NK cells in the uterus during implantation. Often, these theories are used to sell “miracle cures” such as steroids, which can actually compromise the immune system. In fact, treatments that lower the amount of NK cells could damage the uterine NK cells that are beneficial to successful pregnancy.

Blood tests can reveal elevated levels of NK cells in the blood, but scientific evidence can’t definitively link such levels to specific fertility issues. If you have questions about how NK cells may affect your chances of getting pregnant, consult with your doctor. If you want more information about improving fertility or tips for trying to conceive, follow us on Facebook and Twitter, or sign up for our newsletter below.