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What happens when your immune system responds to disease?

Your immune system is your body’s in-built defence system against illness. It works hard to protect you from invading viruses, bacteria, and parasites. But how exactly does your immune system work, and what happens when you get sick?

Here’s a step-by-step look at how your immune response leaps into action.

Your immune system has three main layers of defence. You probably don’t notice some of this happening, even though your body is working hard every day to protect you:

  1. Firstly, your body maintains a healthy and safe barrier that aims to stop germs entering your body. This barrier includes your skin and mucus membranes.
  2. Secondly, your immune system will identify germs, viruses, and bacteria and try to destroy before it can start to reproduce in your body.
  3. Last but not least, if necessary your immune system will work hard to eliminate and get rid of a virus or bacteria which has started to make you sick.

The front line

The front line of your immune system does its best to stop foreign invaders from getting into your body in the first place. However if a virus, bacteria, or parasite still makes its way through, the rest of your immune system will get to work.

The “army” of our immune system is made up of different types of white blood cells, all called defender cells. We make about a billion of these defender cells in our bone marrow every day.

What do the white blood cells do?

Some of these white blood cells - called macrophages - circulate around our body looking for germs and infection. As soon as they see something they don’t recognise, they attack. Your macrophages can tell the difference between your own cells and invading cells because of antigens. Antigens are like ID-tags on the surface of every cell. If a new cell doesn’t carry your unique antigen, then your immune system knows it’s foreign and sees it as a threat.

Did you know that macrophage literally means “big eater”, which is exactly what your macrophage white blood cells do to invading cells?

What’s the difference between a virus and a bacteria?

Your immune system will respond differently to a viral attack and a bacterial infection. Compared to your own body’s cells, bacteria are small and simple. While your body’s cells have a complex structure, including a nucleus, bacteria are just single-cells and around 1/100th the size of a human cell. What makes bacteria so dangerous is that they are independent - they can eat and reproduce very quickly under the right conditions.

In contrast, a virus isn’t alive like a bacteria is. Think of a virus like a little bit of DNA which attaches itself to a cell and puts its own DNA into that cell. It uses the ability of a living cell to reproduce more virus particles.

What happens when the infection fights back?

Sometimes, the virus or bacteria will fight back against the macrophages. This is when the next set of white blood cells comes into action in the form of the more powerful T- and B- lymphocyte cells. Your B-cells make special proteins called antibodies which bind to the virus and stop it from replicating and getting any bigger. They also tag the virus with a special message so other white blood cells can come and destroy them.

Your T-cells help the B-cells out in difference ways. Some of them will put out a kind of alarm call within your body, rallying the troops of other white blood cells. Others will destroy cells which have already been infected by the incoming virus, while some will help the B-cells to produce those important antibodies.

Does your immune system stop once your body is healthy?

The hard work doesn’t end for your immune system. Some of your B- and T-cells will remember the identity of the bacteria or virus they’ve been fighting. This means they can react more quickly if you ever get infected again by the same illness. This is how acquired immunity develops, and can lead to lifelong immunity from just one bout of a specific infection.

There’s a lot going on in your body and it’s no wonder you feel so exhausted when you are ill! Now you know why getting better from an infection seems to take so much energy.

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