What are Natural Killer (NK) cells?

Natural killer cells (NK cells) form part of the body’s immune system. They help the body fight infection and cancer. Every organ has NK cells to protect it, including the uterus (womb).

Natural killer cells (NK cells) form part of the body’s immune system. They help the body fight infection and cancer. Every organ has NK cells to protect it, including the uterus (womb).

NK cells in the uterus are known as uNK cells. They play an important role in helping the body become pregnant.

In order to become pregnant, the womb lining needs to inflame slightly so the embryo can attach (or implant) to the uterine wall. uNK cells contribute to this process of inflammation. If there are too many or too few uNK cells, this can cause too much or not enough inflammation, which are both associated with infertility and miscarriage.

uNK cells also help blood vessels to develop, which make sure that the baby gets a good supply of oxygen and nutrients during pregnancy.

It is possible to have tests to measure your level of NK cells. If you do decide to go ahead with tests or treatment, please be aware that the treatment is still considered experimental. Although there is a series of scientific studies that find that NK cells are important for normal pregnancy, it’s still not clear what the optimal level of uNK cells is and what is the best way to correct any imbalance. This is still yet to be tested in large scale studies. 

Can I get tested to check the level of my NK cells?

It is possible to have tests to measure your level of NK cells. This is not usually available on the NHS, although some women have told is that they have had these tests through their NHS recurrent miscarriage clinic. Some fertility clinics offer tests, but not all. If they do, you will have to pay for it. This can be expensive and will vary from clinic to clinic.

Before deciding whether or not to have tests, it’s important to know that there are some issues with these tests:

  • There is a lack of evidence about the exact role that NK cells have in causing miscarriage.
  • There are no official guidelines for what ‘normal’ NK cell activity is.
  • It is difficult to measure the ideal level of NK cells and when an imbalance can cause infertility and miscarriage.
  • Specialists have different opinions about how to do these tests and report the results – as there are no official guidelines, doctors will interpret the results based on their clinic’s ‘normal’ range and their clinical and professional experience.
  • The level of uNK cells is different in each menstrual cycle so having a single test may not give a clear picture.

Testing may involve peripheral NK cell testing. This is a blood test that measures the percentage and quantity of NK cells in the bloodstream. However, these cells are different to uNK cells. Therefore, some clinics do uNK testing, which is similar to an endometrial scratch. This involves scratching the lining of the womb (the endometrium) to test the tissue for NK cell activity.

Is there treatment for abnormal levels of NK cells?

Treatment for abnormal NK cells is usually aimed at suppressing the number and activity of the uNK cells. The problem with this approach is that having too few or too many uNK cells have both been associated with recurrent miscarriage. Treatment for abnormal NK cells includes medications that are often used to treat conditions that affect the immune system (known as autoimmune diseases). This includes prednisone, which is a steroid commonly used for asthma and arthritis.

Intravenous immunoglobulin (IVIg) may also be given intravenously during pregnancy. This means it is given by a drip into the vein. Immunoglobulins are proteins produced naturally by the body's immune system to fight off infections.

It’s important to know that the Royal College of Obstetricians and Gynaecologists (RCOG) says that there is not enough evidence that these treatments are effective. These medications can also have serious side effects in pregnancy. For example, prednisolone can affect the baby’s growth in the womb.

Who can I talk to about NK cells?

You can talk to your doctor about NK cells to find out more. They can give you their professional opinion about whether testing and treatment may help you. Doctors in different clinics may have different opinions.

Brighton, P et al (2017) Clearance of senescent decidual cells by uterine natural killer cells in cycling human endometrium. eLife https://elifesciences.org/articles/31274

Lucas, ES, Vrljicak, P., Muter, J et al (2020) Recurrentl pregnancy loss is associated with a pro-senescent decidual response during the peri-implantation window. Communications Biology 3, 37 (2020) https://www.nature.com/articles/s42003-020-0763-1

Royal College of Obstetricians & Gynaecologists (2016) The Role of Natural Killer Cells in Human Fertility https://www.rcog.org.uk/globalassets/documents/guidelines/scientific-impact-papers/sip_53.pdf

Review dates

Last reviewed: 5 June 2020
Next review: 5 June 2023