Our own 'killers' inside the body: Let's simply learn about Natural Killer Cells (NK Cells)

You probably know that our bodies have an amazing defense system, like an army, to fight diseases. We call this the immune system. There are different types of soldiers in this army. Some spy, some make weapons, and others go straight into battle. Today we are going to talk about a very special, slightly different, and very skilled group of commandos in this army. Those are the 'natural killer cells' or as we all know them, Natural Killer Cells (NK Cells) . The name sounds a little scary, doesn't it? But these are actually some of our best friends in our bodies.

Who are these Natural Killer Cells (NK cells)?

Simply put, Natural Killer Cells (NK cells) are a type of white blood cell in our body. To be precise, they belong to a group of cells called lymphocytes, which are a major type of cell in the immune system. You may have heard of B-cells and T-cells. That's what NK cells are all about.

But there's a reason they're called "natural" killers. Other fighter cells in our immune system, like cytotoxic T-cells, need to be familiar with an enemy (like a virus or bacteria) in order to destroy it. That means they need to have some training and memory about that enemy.

But our speakers, `NK cells`, do not need any prior training or recognition. They can destroy any cell that might pose a threat to them as soon as they see it, without any prior experience. They are like a smart guard dog that instinctively recognizes a thief and jumps on him when a thief comes into a house.

What exactly happens with these NK cells?

The main function of `NK` cells is to destroy cells that were previously healthy but now pose a threat to our body. This mainly includes:

• Virus-infected cells: When a virus enters the body, it enters our healthy cells and takes control of them. `NK` cells can recognize these virus factories and destroy them before the virus can spread further.
• Cancer cells: NK cells have the amazing ability to recognize and destroy some cancer cells that develop in our bodies at an early stage. Therefore, these cells play a great role in preventing the spread of cancer.

These `NK` cells are a very important part of our innate immune system. This innate immune system is our body's first line of defense. Think of our body as a fortress. The walls of that fortress (our skin) and the moats (mucous membranes) are the first things that prevent germs from entering. But if an enemy somehow gets past those barriers and enters, the first army unit that goes into battle is this innate immune system. The best soldiers in that team are the `NK` cells.

Okay, so how do these NK cells work?

This is a very interesting process. `NK` cells are constantly patrolling our bodies. As they do so, they "scan" every cell they encounter, asking, "Is this a healthy cell of ours or a dangerous cell?" The decision whether an `NK` cell will destroy a cell or not depends on the signals it receives or does not receive from the cell it is scanning (let's call it the `target cell').

On the surface of an NK cell, there are many receptors that both activate and inhibit the destruction of a cell.

How do we recognize healthy cells and avoid damaging them? (Inhibition)

Almost every healthy cell in our body has an ID card on its surface that says, "I am a healthy cell that belongs in this body." This is called MHC-1 . When an NK cell goes on patrol, when it encounters a cell, it checks for this MHC-1 ID card.

If the ID is correct, meaning the cell is healthy, the MHC-1 binds to the inhibitory receptor on the NK cell. It's like a guard looking at the ID and saying, "Okay, you can go." At that point, the killing process of the NK cell simply turns off. So the NK cell goes to check the next cell without harming the healthy cell.

How do you decide to destroy dangerous cells? (Activation)

An `NK` cell enters the fight in situations like these:

• When the ID card (MHC-1) is lost: Cells infected with a virus and some cancer cells hide this ID card called `MHC-1`. That is to avoid the immune system. But `NK` cells are very cunning. As soon as they see a cell without an ID card, they decide that it is suspicious and attack it.
• When the identity card (MHC-1) decreases: Sometimes a cell's `MHC-1` is not completely lost, but the amount decreases. That is also a warning signal to `NK` cells.
• When Activating Signals Are Received: Some infected cells and cancer cells emit special signals on their surface that say, "I am dangerous." When these signals are picked up by the 'activating receptors' on NK cells, the killing process begins.

After deciding to destroy a cell, the NK cell uses two very powerful chemical weapons.

1. Perforin: This creates a hole on the outside of the target cell.

2. Granzymes: These enzymes called `granzymes` are sent through the hole that was made. Once these enter, the target cell is destroyed from the inside.

After completing this task, the NK cell sends messages (via cytokines) to others to come and clean up and see if there are any more enemies.

Where are these NK cells located in our body?

NK cells first develop in our bone marrow. That's the spongy part inside our large bones. After developing there, some NK cells stay in the bone marrow. Others travel to various organs connected to our lymphatic system. For example:

• Lymph nodes
• Spleen
• Tonsils
• Thymus

Once these mature into NK cells, they are released into the bloodstream. That's why there are so many of these cells in our blood. In addition, these NK cells can also be found in organs like the liver and lungs. Normally, there are more than 2 billion NK cells in the body of an adult at any given time. However, their lifespan is a bit short, about two weeks.

What is the difference between NK cells and Cytotoxic T-cells?

Although both of these are killer cells in the immune system, their function and function are different. To understand this difference clearly, see the table below.

What diseases are caused by problems with NK cells?

Not having enough NK cells in our bodies, or having the cells that we have not function properly, is a major threat to our health, as it reduces our ability to fight infections and cancer.

Research has found that changes in the function of NK cells can cause various diseases.

• Autoimmune diseases: In these diseases, our immune system attacks our own healthy cells. It is believed that the dysfunction of `NK` cells may contribute to this condition.
• Asthma: NK cells contribute to controlling or increasing inflammation in the airways in asthma.
• Inflammatory Bowel Disease (IBD): NK cells are involved in the body's inflammatory response to bacterial infections in the digestive tract.

In addition, there are two inborn genetic defects that are specific to NK cells.

1. Classical Natural Killer Cell Deficiency (NKD): People with this condition have very few or no NK cells in their blood.

2. Functional Natural Killer Cell Deficiency (NKD): Here, although there are sufficient `NK` cells, the function of those cells, that is, their ability to kill cells, is very weak.

People with both conditions are at increased risk of developing frequent, especially viral infections. For example, HPV, herpes, cytomegalovirus (CMV), and the virus that causes chickenpox can have a significant impact.