Natural killer cells (NK cells)


Natural killer cells, often called NK cells, are a type of lymphocyte, which is a white blood cell that plays an important role in the body’s immune system. They are part of the innate immune system, meaning they can respond quickly to threats without needing prior exposure to the specific invader.

NK cells are called “natural killers” because, unlike other immune cells such as T cells and B cells, they do not require prior activation to recognize and kill abnormal or infected cells. This allows them to act quickly as a first line of defense against infection and cancer.

What do natural killer cells do?

The main job of NK cells is to identify and destroy cells that are abnormal, infected, or cancerous. They are especially good at recognizing:

  • Virus-infected cells.

  • Cancer cells.

  • Cells that are under stress or have abnormal surface proteins.

NK cells do this by detecting specific proteins, called ligands, on the surface of other cells. These ligands may be missing or altered in unhealthy cells, allowing NK cells to tell the difference between normal and abnormal cells. Once a target is recognized, NK cells release substances that cause the abnormal cell to break apart and die, a process called apoptosis.

NK cells also play a role in regulating other parts of the immune system by releasing chemical signals (cytokines) that attract and activate other immune cells.

Where are natural killer cells found in the body?

NK cells are found in the bloodstream, lymph nodes, bone marrow, spleen, and other tissues throughout the body. They travel through the blood and lymphatic system, constantly looking for abnormal cells to eliminate.

How are natural killer cells different from other lymphocytes?

NK cells are part of the innate immune system, while other lymphocytes like T cells and B cells are part of the adaptive immune system. This means:

  • NK cells can respond immediately to threats without needing to recognize a specific virus or bacteria.

  • T and B cells must first be trained to recognize a particular invader before they can respond effectively.

Despite these differences, NK cells and other lymphocytes often work together to protect the body.

Can natural killer cells be involved in cancer?

Yes. While NK cells usually help prevent cancer by destroying abnormal cells, in rare cases NK cells themselves can become cancerous. These types of cancer are called NK cell neoplasms, and they often involve both NK cells and T cells. These include:

  • Extranodal NK/T-cell lymphoma – A rare type of non-Hodgkin lymphoma that usually starts in the nose, sinuses, or upper airway, but can spread to other parts of the body. It is often linked to Epstein-Barr virus (EBV) infection.

  • Aggressive NK cell leukemia – A very rare and rapidly growing blood cancer made up mostly of NK cells. It usually affects adults and often progresses quickly with a poor prognosis.

  • Chronic lymphoproliferative disorder of NK cells – A slow-growing disorder where NK cells multiply over time. It is usually diagnosed through blood tests and may not cause symptoms for many years. In some cases, it may progress to a more aggressive NK cell cancer.

These conditions are rare, and while they are associated with NK cells, they may also involve other types of immune cells, such as T cells.

How do pathologists test for NK cells?

Pathologists can identify NK cells by using a technique called immunohistochemistry or flow cytometry. These tests look for proteins on the surface of the cells that are commonly found on NK cells, such as CD56, CD16, and NKp46. These tests can help confirm whether a group of abnormal cells in a tissue or blood sample are NK cells.

These markers are especially helpful when diagnosing NK cell neoplasms or when trying to understand the immune environment around a tumour.

Questions to ask your doctor

  • What role do NK cells play in my diagnosis?

  • Were NK cells found in my tissue or blood sample?

  • Do I have a condition involving abnormal NK cells?

  • Are additional tests (like immunohistochemistry or flow cytometry) needed?

  • What does this mean for my treatment or follow-up plan?

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