Natural killer cells (NK cells) are white blood cells that destroy infected and diseased cells, like cancer cells. They’re also a type of lymphocyte, like B-cells and T-cells. NK cells can destroy harmful cells in the early stages, preventing viruses and cancer cells from spreading.
Natural killer cells (NK cells) are white blood cells that destroy infected cells and cancer cells in your body. NK cells are important fighters in your immune system. Your immune system protects you from harmful invaders, like pathogens (viruses, bacteria and parasites) and cancer cells.
NK cells are called “natural” killers because they can destroy potential threats without prior exposure to a particular pathogen. Other lymphocytes that destroy harmful cells, like cytotoxic T-cells, need previous exposure to a pathogen before they can destroy it.
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NK cells destroy once healthy cells that now pose a threat. This includes cells infected with viruses and cells that have become malignant (cancer cells). NK cells destroy these harmful cells in the early stages, preventing spread.
Natural killer cells are fighters in your innate immune system, which is an essential part of your immune system. Your innate immune system is your body’s first-line defense against all threats. It includes physical barriers, like skin and mucous membranes, which keep germs out. It includes select immune cells, like NK cells, that destroy threats that have made it past your body’s barriers.
While an NK cell’s primary job is killing, these cells also communicate. They release proteins called cytokines that direct other cells in your immune system to attack harmful cells and pathogens.
Natural killer cells patrol your body, scanning cells for markers that indicate healthy or diseased cells. If they detect signs a cell is harmful, they release deadly chemicals into the cell to kill it. Whether or not an NK cell kills depends on the signals it does (or doesn’t) receive from the cell it’s scanning (the target cell).
The surface of an NK cell contains multiple receptors that work together to either activate or inhibit (prevent) an NK cell from killing a target.
Natural killer cells don’t attack cells with markers that indicate they’re healthy cells that belong in your body. MHC-1 is the most common cell marker an NK cell recognizes as belonging or “self.” The MHC-1 on the target cell attaches (binds) to an NK cell’s inhibitory receptor. The binding turns the NK cell’s killing function off.
Instead of attacking, the NK cell moves on to the next cell.
Natural killer cells activate to destroy target cells they don’t recognize as belonging. These include:
NK cells release perforin and granzymes to kill a target cell. Perforin creates an opening in the target cell so the NK cell can insert granzymes. The granzyme kills the cell.
Activated NK cells release cytokines that tell other white blood cells to help rid your body of the threat.
NK cells start developing in the spongy tissue inside some bones called bone marrow. As they continue to develop, NK cells may stay in your bone marrow. They may move to other tissue and organs in your lymphatic system, such as your:
About 5% to 10% of the lymphocytes circulating in your blood are NK cells. They have a short lifespan of about two weeks. At any given time, adults have more than 2 billion NK cells.
Not having enough NK cells leaves you vulnerable to infections and cancer. Research suggests that NK cells may also play an important role in other conditions, including:
Two immunodeficiency conditions are specific to NK cells. Both result from genetic mutations present at birth. A genetic mutation is a change in your DNA, the instructions that tell your cells how to work.
Both conditions make you more susceptible to infections, including:
NK cells and cytotoxic T-cells are lymphocytes that destroy harmful cells. But they serve different roles in your immune system.
Natural killer cells function based on the signaling processes that involve their activating and inhibitory receptors. They activate when signals that prevent them from attacking (like MHC class I molecules) are absent or downgraded on a target cell.
Natural killer cells are an essential part of a healthy immune system. Unless you have classical NKD, a genetic condition that prevents your bone marrow from making NK cells, you have NK cells.
A note from Cleveland Clinic
NK cells protect you from infection and diseases by responding to threats quickly. They destroy harmful cells in the early stages, preventing infected cells and cancer cells from spreading. Having a normal amount of healthy NK cells is an important part of a healthy immune system.
Last reviewed by a Cleveland Clinic medical professional on 04/10/2023.
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