Natural killer cell

Your body has tiny guards.

They look for bad cells. These guards act fast to help you. They stop germs from making you sick.

Some guards look for cells that are sick. They find cells with a virus inside. They also find cells that are growing too fast.

These guards are special. They can find bad cells that other guards miss. They do this by looking for a special sign. If the sign is gone, the guard knows it is a bad cell.

The guards use tiny tools to work. They can make small holes in a bad cell. This helps stop the bad cell from working.

These tiny guards are always ready to help. They keep your body safe and strong.

Your body has special guards called natural killer cells. We often call them NK cells. They are part of your innate immune system. This system helps you fight germs right away.

NK cells are very smart. Most guards need a signal to start working. But NK cells can act on their own. They look for cells that are missing a special sign. We call this sign MHC class I. Healthy cells always show this sign.

Sometimes, bad cells hide by losing that sign. Viruses or tumors might make the sign go away. Other guards might miss these sneaky cells. But NK cells see them. They know a cell is bad if the sign is gone.

When an NK cell finds a bad cell, it attacks. It uses tiny parts called granules. These granules hold special tools. The cell lets out a tool called perforin. Perforin makes tiny holes in the bad cell. Then, it lets out tools called granzymes. These tools go through the holes. They cause the bad cell to break down. This helps stop the sickness from spreading.

Natural killer cells, or NK cells, are vital parts of your innate immune system. They are a type of large granular lymphocyte, which is a kind of white blood cell. These cells make up about 5% to 20% of all lymphocytes in a human body. They act as rapid responders to help fight off many different threats. NK cells can spot cells that have been infected by a virus. They also find stressed cells or tumor cells that might cause harm.

NK cells work by checking for specific signals on the surface of other cells. Most immune guards need a special signal to start their work. However, NK cells are different because they look for what is missing. Healthy cells show a marker called MHC class I on their surface. Some bad cells try to hide by getting rid of this marker. When an NK cell sees a cell lacking this sign, it attacks. This is why they are called "natural killers."

Scientists have been studying these cells for a long time. In 1966, Dr. Henry Smith at the University of Leeds found that some cells could fight tumors naturally. Later, in the early 1970s, researchers like Rolf Kiessling and Hugh Pross discovered the specific cells responsible. They worked at the Karolinska Institute in Stockholm to study these findings. By 1980, Timonen and Saksela used microscopes to actually see these cells for the first time. This was a major breakthrough for everyone studying the immune system.

There are different types of NK cells with different jobs. Some are called CD56bright cells and are often found in the liver or skin. These cells mostly release signals to help other parts of the immune system. Other cells are called CD56dim cells and move through your blood. These cells are very good at direct cell killing. They use a marker called CD16 to help them target specific threats. NK cells can also be identified by markers like CD56 or NKp46.

When an NK cell decides to attack, it uses tiny tools called granules. These granules contain proteins named perforin and granzymes. The cell releases perforin to poke small holes in the target cell. This is like making a tiny door in a wall. Then, the granzymes enter through those holes to destroy the cell. This process can lead to a controlled breakdown called apoptosis. This method is very helpful because it destroys the threat without spreading viruses.

Natural killer cells, often called NK cells, are specialized white blood cells. They belong to a group of cells known as large granular lymphocytes (LGL). They are also part of the innate lymphoid cell (ILC) family. These cells make up about 5% to 20% of all circulating lymphocytes in humans. NK cells are critical to the innate immune system. They provide a rapid response to threats like virus-infected cells and tumor cells. They also target stressed cells and other intracellular pathogens.

NK cells work through a complex balance of signals. They use activating receptors to find targets and inhibitory receptors to protect healthy cells. Most immune cells, like cytotoxic T cells, require a specific antigen to be presented on MHC class I molecules. However, some harmful cells hide by removing these MHC class I markers. NK cells are unique because they recognize these "missing self" signals. If a cell lacks these markers, the NK cell is triggered to attack. This ability to kill without prior sensitization is why they are called "natural killers."

Scientists classify NK cells into two main types based on their surface markers. The first type is known as CD56bright NK cells. These cells are mostly found in the bone marrow, liver, and skin. They primarily influence the immune system by releasing cytokines. They also tend to target highly proliferative cells, which may help regulate the immune response. The second type is the CD56dim NK cells. These are found mostly in the peripheral blood. They are highly effective at direct cell killing. They are also CD16 positive, which helps them perform antibody-dependent cellular cytotoxicity (ADCC).

When an NK cell identifies a target, it uses a precise biological mechanism to destroy it. The cell contains small granules filled with proteins like perforin and granzymes. When the NK cell reaches a target, it releases these granules. Perforin works by forming pores, or tiny holes, in the target cell membrane. This creates a channel that allows granzymes to enter the cell. Once inside, the granzymes induce apoptosis, which is a programmed form of cell death. This is safer than simple cell lysis because it destroys the threat without spreading viruses.

Research into these cells has a long and fascinating history. In 1966, Dr. Henry Smith at the University of Leeds observed that some cells could destroy tumors naturally. In the early 1970s, researchers Rolf Kiessling and Hugh Pross identified the specific cells responsible in mice. At the same time, Hugh Pross and Mikael Jondal discovered these cells in humans. These studies were supervised by professors Eva Klein and Hans Wigzell at the Karolinska Institute. In 1980, Timonen and Saksela achieved a major breakthrough. They were the first to visualize NK cells using a microscope.

NK cells use many different receptors to communicate with their environment. Natural cytotoxicity receptors (NCR) help mediate killing and the release of interferon gamma. Toll-like receptors (TLR) also play a role by recognizing patterns from pathogens. NK cells express various levels of these receptors, such as high levels of TLR-1. Inhibitory receptors, like the Killer-cell immunoglobulin-like receptors (KIR), protect healthy cells. KIRs recognize the MHC class I molecules on normal cells to prevent accidental attacks. This ensures the immune system only targets cells that are truly dangerous.

Understanding NK cells is vital for modern medicine. Scientists are currently researching how to use NK cell activity for new treatments. This includes potential therapies for cancer and HIV. Because NK cells can adjust to their environment, they may even form immunological memory. This allows them to respond more effectively to repeat infections. Their dual role in both innate and adaptive immunity makes them a major focus of biological study. They represent a bridge between immediate defense and long-term protection.