Natural Killer Cells: Definition & Functions

Instructor: Catherine Paul

Catherine has taught high school science and has a master's degree in biology.

Discover what makes natural killer cells such an important part of our immune system. Learn about the characteristics of natural killer cells and how they defend against tumors and viruses.

Natural Killer Cell Definition

Quick to react, super-speed, and the strength to kill invaders - nothing short of a superhero. I'm not talking about Superman, but pretty close - natural killer (NK) cells. These cells are part of our immune system, and we are lucky we have them. They are our body's first line of defense, the first cells to respond against viruses and tumor cells.

NK cells are part of a group of lymphocytes called null cells. Unlike other lymphocytes, such as T and B cells, they do not have to find their specific match to identify an invader. Similarly, they do not depend on memory of past pathogen infections to attack a cell. They are 'null' of both specificity and memory, allowing them the freedom to react to any tumor cell or cell infected with a virus. This is why NK cells are nothing short of our immune system's superhero.

NK Cells Have Special Characteristics

Although natural killer (NK) cells are technically lymphocytes, a lot about them distinguishes them from other cells in this group. They mature in the bone marrow instead of the thymus like other T cells. T cells use CD4 or CD8 on their surface membrane to identify a specific cell infected with a pathogen. Each T cell is limited to identifying only targets that match with their surface receptors. However, NK use the CD16 on their cell membrane to recognize and bind to any tumor cell or virally infected cell. It is not restricted to a specific pathogen. Always on guard, NK cells do not rely on the memory of exposure to a specific pathogen, known as 'immunity', to attack an intruder.

This immune system superhero has a utility belt filled with secret weapons called perforin and granzymes. They are hidden away inside the large NK cells within granules. When the NK cell meets its nemesis it binds to it and releases perforin and granzymes, which puncture holes in the membrane of the tumor cell or cell infected with the virus. This results in cell death by apoptosis.

Natural Killer Cell Morphology CAPTION=

Target Recognition

How does our 'superhero' NK cell know when a cell is infected with a virus or has become cancerous?

Imagine our NK cell vigilantes flying around, cape billowing in the wind, scouting for invaders. All cells must wear a 'badge' to let the immune system know that they belong. NK cells circulate through our bodies, checking 'badges'. This 'badges' are called a major histocompatibility complex molecules, or MHC molecules for short. All nucleated cells display class I MHC molecules on their cell membranes.

If a tumor cell or cell infected with a virus is circulating in our system, it will decrease its class I MHC molecules. As NK cells travel through our bodies, they bind to class I MHC molecules. If the 'badge' is normal, the NK cell sees no threat and moves on. When a cell displays less than the normal amount of class I MHC molecules, the NK cells become activated when they bind to the cell. Perforin and granzymes are released by the NK cells to kill the infected cell.

NK Cell Attaches to Target Cell caption=

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