What Are NK Cells and How Does Immune Surveillance Work?

Hello, I am Dr. Hidenori Shimada of Grand Green Osaka Umekita Clinic in Osaka, Japan.

If you are researching cancer immune cell therapy, you will quickly meet the term NK cells. Understanding what these cells do, and what the idea of immune surveillance means, makes it easier to read information about NK cell therapy with realistic expectations. This article explains the basics in plain language for patients in Japan and abroad, and it lists questions worth bringing to a doctor. It is general information, not medical advice.

The short answer

NK cells, short for natural killer cells, are a type of lymphocyte that circulates in the blood and can detect and attack abnormal cells, including cancer cells and virus-infected cells, without waiting for instructions from other immune cells. Immune surveillance is the term for this ongoing patrol. NK cell therapy is an approach in which a patient’s own NK cells are activated and expanded outside the body and then returned by intravenous infusion, with the aim of reinforcing immune function. It is a treatment option to learn about, not a replacement for standard cancer care, and whether it is appropriate for you is a decision for a physician after a consultation.

What are NK cells?

The immune system has many types of cells. Some, such as T cells, need to be shown a specific target before they act. NK cells belong to the innate immune system, which means they can respond quickly and on their own. They patrol the body and look for cells that appear abnormal, and they can attack such cells directly using molecules such as perforin.

Researchers have studied NK cells for decades, and the biology is well described in the scientific literature. How much that biology translates into benefit for an individual patient is a separate question, and it depends on the cancer type, the stage, previous treatment and many other factors. For that reason, a responsible discussion of NK cells always separates what the cells do in the laboratory from what a therapy may or may not achieve in a person.

Cancer and immune surveillance

Even in healthy people, abnormal cells that could develop into cancer are thought to appear regularly. Not every abnormal cell becomes cancer, partly because immune cells, including NK cells, find and remove many of these abnormal cells. This mechanism is called immune surveillance. When surveillance weakens, abnormal cells may be able to slip through and multiply more easily.

NK cell activity is known to decline with age, with stress, and with the effects of cancer itself. This is one of the reasons why researchers became interested in cell therapies that return larger numbers of active NK cells to the body. It is important to remember that this is a rationale for the approach, not proof that it works for every person. Cancer is a complex disease, and tumors can find ways to avoid immune attack.

How NK cell therapy works

At Grand Green Osaka Umekita Clinic, NK cell therapy is described as an immune cell therapy that uses your own cells. The general flow is simple to follow:

• A standard blood draw provides the starting cells.
• The NK cells are activated and expanded at a licensed cell processing facility.
• The larger and more active population of your own NK cells is returned to your body by intravenous infusion.

Because the cells come from you, the risk of rejection is considered low. Temporary reactions such as fever or fatigue may occur, and every treatment carries some risk, so you should ask the doctor to explain possible side effects in detail. Treatment usually does not require hospitalization, but the exact schedule and number of visits should be confirmed during your consultation.

How it fits with standard cancer treatment

NK cell therapy is not presented as an alternative to surgery, chemotherapy or radiation therapy. Information from the clinic states that it can often be used alongside these treatments, depending on the individual case, and the clinic can coordinate with the medical institution that provides your standard treatment. Some people also consider it after treatment, with the aim of supporting recurrence prevention or maintaining immune function.

If you are currently being treated, please tell your oncologist that you are considering an immune cell therapy. Open communication between all of your doctors helps each of them make better decisions. Never stop or delay standard treatment because of something you read online, including this article.

Legal framework, fees and what to ask a doctor

In Japan, NK cell therapy is provided as a self-pay treatment, which means it is not covered by national health insurance. It is offered under Japan’s Act on the Safety of Regenerative Medicine, which requires a treatment plan to be submitted to and accepted by the authorities. This system is intended to support safety management, but it does not guarantee outcomes. Costs, risks, side effects and eligibility must all be confirmed at a medical consultation.

Questions you may wish to prepare include:

• Is NK cell therapy appropriate for my type and stage of cancer?
• How would it be coordinated with my current or planned standard treatment?
• What are the possible side effects and risks in my situation?
• What are the total fees, and what is included?
• How many visits are needed, and how long should I plan to stay in Osaka?

Grand Green Osaka Umekita Clinic is located in the Umeda area of Osaka and provides information pages in several languages, including English, for international patients. If you are interested, you can start by reading the NK cell therapy page and contacting the clinic through its web or LINE channels to ask about a consultation. A consultation is a place to ask questions and understand your options, and it does not commit you to treatment.

1. 1ConsultationDoctor reviews your situation
2. 2Blood draw
3. 3Activation and expansionAt a licensed facility
4. 4IV infusionYour own NK cells returned

This figure is a conceptual diagram that summarizes the article

References

• PubMed: Regenerative lineages and immune-mediated pruning in lung cancer metastasis. (2020)
• PubMed: Natural Killer Cells: Development, Maturation, and Clinical Utilization. (2018)
• PubMed: Exploiting innate immunity for cancer immunotherapy. (2023)
• PubMed: Glioblastoma stem cell metabolism and immunity. (2024)
• PubMed: NK cell-based cancer immunotherapy: from basic biology to clinical development. (2021)
• PMC: Platelet-Mediated Protection of Cancer Cells From Immune Surveillance – Possible Implications for Cancer Immunotherapy. (2021)
• PMC: Is the MAPK ERK5 the nexus from FAO to NK cell-mediated metastasis immune surveillance? (2025)
• PMC: Oncogenic stress sensed by the immune system: role of natural killer cell receptors. (2009)
• MHLW (Japan): Regenerative Medicine — Ministry of Health, Labour and Welfare, Japan

Supervising Physician

Director, Medical Corporation Shimada Clinic
Head of Clinical Practice, Grand Green Osaka Umekita Clinic

Hidenori Shimada

Graduated from the University of Tokushima Faculty of Medicine in 2005. After his initial residency, he obtained U.S. medical certification (ECFMG) and trained as a sub-intern in general surgery at the University of Washington. After completing the doctoral program at Kyoto University Graduate School of Medicine and working as a researcher at the Center for iPS Cell Research and Application (CiRA), Kyoto University, he founded Shimada Clinic in 2013. At Grand Green Osaka Umekita Clinic (https://umekitaclinic.org), he is engaged in a wide range of care, from internal medicine and pediatrics to cell therapy and aesthetic medicine.

* The physician supervision of this article covers its academic content only and does not constitute a recommendation of any specific treatment or product.