Fighting infection

Being sick is no fun, but what's your body doing while you feel awful? Viki Male has the answer...

Your body is continuously under attack by microscopic living things that can do you harm, called pathogens, like bacteria, viruses and parasites. There are barriers, your body’s castle walls – like your skin and the acid in your stomach – that stop them getting in, but sometimes they can breach the castle walls and then it’s the job of the immune system, to defend you.

The cells of the immune system are found all over the body, but they’re also in almost every other organ you have.

One of the most useful things the immune system can do is to remember nasty things, so that the second time it encounters an invader, it responds more quickly.

This happens because of two very special kinds of immune cells, called T and B cells. T cells and B cells  are unique among all the cells in the body, because they randomly generate receptors, which are the bits of themselves that they use to recognise and stick to other molecules. They’re guessing at all the things that might hurt you. And they make lots and lots and lots of guesses.

Your body can make T cells with up to 30 sextillion different receptors – that’s about the same as the number of stars in the observable universe - so for any molecule you can imagine there’ll be a T and a B cell that can stick to it.

But of course, you don’t want to make an immune response to all everything in the whole world, all the time. Hay fever is your immune response attacking the ordinary, harmless pollen that gets into your nose.
So how do we control the immune response so that it’s only switched on when it should be, and isn’t like that house alarm that goes off when the neighbour’s dog thinks about barking?

Let’s imagine that you fall off your bike and graze your knee, and some bacteria get in. As a first line of defence, those bacteria will be swallowed by immune cells called macrophages.

That makes it sore, hot, red and swollen. These things are actually side-effects of the macrophages recruiting other immune cells to help clear the infection.

Some of the bacteria will be eaten by another kind of immune cell, called a dendritic cell. Dendritic cells can tell when they’ve eaten something dangerous because they have bits inside them called pattern recognition receptors, which are only activated by pathogens.

So the invader activates the dendritic cell, and then the dendritic cell travels away from the site of injury, through a series of tubes in your body called lymphatic vessels, until it reaches a big junction, called a lymph node.

When it gets there, it shows fragments of the bacteria, the leftovers of its bacterial lunch, to all the T cells; even though only some T cells will have the right receptor, these ones will be able to recognise those fragments.
And alongside the fragment, the dendritic cell also gives the T cell a “danger signal”, which tells the T cell that what it’s seeing comes from a something that we would rather not have be in the body.

This switches on the T cell, and the T cell can then switch on B cells – but only those B cells that have a receptor that binds to the same molecule (in this case, our bacterial molecule) as the T cell.

The activated B cells make a version of their receptor called an antibody, which travels in the blood and attaches to the bacteria.

These are like big neon signs which tell macrophages “FREE LUNCH HERE”, which helps macrophages to eat them, speeding up the clearance of the infection.

At the end of the immune response, most of the T and B cells die, but a few veterans will survive for the rest of your life. They’re called “memory” cells.

If you’re ever infected by the same bacteria again, they can rapidly divide and make antibody so the infection can be cleared more quickly - in most cases so quickly you won’t even know you’re infected.

We’ve got a lot to thank our immune systems for.