There is so much of you, that  if your cells were human-sized,  

you would be as big as 20 Mount Everests. For your  creepy-crawly inhabitants, this makes your body an  

ecosystem, rich in resources and warmth and space.  A perfect place to move into and have a family.  

While some of these guests are welcome, most  are not. Your immune system is the guardian  

of this planet, the force tasked with protecting  yourself against the constant danger of invasion.

Unfortunately your enemies in the  tiny world have a huge advantage.

Consider the effort it takes to make a single copy  of yourself and your trillions of cells. First you  

need to find someone who thinks you are cute,  date and be awkward and if things work out in a  

complicated dance, two of your cells merge  together. Then you need to wait for months  

while the cell multiplies over and over until  it is released into the world as a human being.  

And even then you only have a mini copy  that needs years to become remotely useful.

A bacterium consists of one cell. It can make  a fully grown copy in about half an hour.  

A virus can turn into hundreds within  hours and billions within days.

Your enemies multiply orders  of magnitude faster than you.  

Even worse, for a bacterium or virus  your body is a hostile ecosystem  

applying selective pressure. Because they go  through so many generations so quickly, eventually  

by pure chance, there will be an individual  that mutates and adapts in just the right way  

to resist your defenses and then multiply quickly  again. In other words, you are facing a sheer  

endless variety of different enemies and you’re  too slow to keep up with their evolution.

This is bad.

Luckily your immune system is just about  the most amazing thing ever. The second most  

complex biological system known to us, after the  human brain, and so sophisticated that we still  

haven’t discovered all its secrets. Since it is  so complicated we have to simplify and focus on  

one thing at a time. If you want the full story,  wait for the announcement at the end of the video!

Ok, so why are we not all killed by some  new bacteria or virus? In a nutshell,  

you actually have two immune systems, the  innate and the adaptive immune system.  

The innate immune system was  ready when you were born.  

It mostly consists of general purpose soldiers,  we introduced them in the last immune video.

The adaptive immune system carries two types of  cells T Cells and B Cells that are your super  

weapons and are incredibly effective and deadly  for your enemies. These cells are complicated to  

produce and take a lot of time to deploy but  once they are ready, they pack a real punch.

What makes your adaptive immune system  so powerful is that it has the largest  

library in the universe. It has an answer to  everything. You have at least one of these  

super weapon cells inside you to fight the  black death, the corona virus and the first  

deadly bacteria that will emerge in a city  on Mars in one hundred years. This makes it  

possible for you to counter the ability of  bacteria and viruses to change so rapidly.

How is this possible? To understand what is  going on here, we need to take one step back.

All organisms on earth are  made from the same basic parts,  

mostly proteins. Proteins are  the building blocks of life  

and can have billions of different shapes  - you can imagine them as 3D puzzle pieces.  

There are billions of different puzzle pieces  your enemies can use to construct their bodies.

Why is this important? Because proteins are  in a way the “language” of the microworld.  

Cells don’t have eyes or ears, so to tell friend  from foe, they have to touch them and recognize  

if their protein is part of a friend or part of an  enemy. Recognizing means that cells have countless  

tiny devices called receptors, that are able to  connect with a specific protein puzzle piece.  

So your cells have tiny puzzle pieces on their  outsides that are able to click together,  

or recognize other protein puzzle pieces.  When a cell connects together with a protein  

and recognizes it as “enemy”,  it knows that it has to attack.

Only if your cells can make this  distinction between friend or foe  

is your immune system able to fight an invader.

But since there are billions of  possible protein puzzle pieces,  

this means there are billions of possible enemy  puzzle pieces. This is also one of the reasons  

we still have to deal with diseases like the  flu each year – the influenza virus mutates  

very rapidly and so the proteins that make  up its hull constantly change a tiny bit.

The soldiers of your innate immune system have  a large number of the puzzle pieces for common  

bacteria and viruses memorized, that’s why  they are your all purpose weapons. But they are  

ineffective against many billions of mutations  and adaptations that your enemies can develop.

So the reason you are still alive is that your  Adaptive immune system is able to recognize  

between one billion and ten billion different  enemy protein puzzle pieces, which is enough  

to be prepared for every possible enemy. But  how is this possible? How could your immune  

system possibly have this much of a variety to be  prepared for every possible protein puzzle piece?

Well, the cells of your adaptive immune system  found a cheat code: mixing and matching their own  

genetic code to create this stunning variety of  receptors. The details are way too complicated for  

this video but in a nutshell, your adaptive immune  cells have official permission to take a tiny part  

of their own genetic code and mix it in random  ways to create billions of different receptors.

A good way to explain this is by asking  you to imagine an army of cooks, with each  

of them wanting their own special recipe. They  have 100 different ingredients to choose from.  

Each ingredient stands for one tiny  piece of genetic code in this metaphor.

So each cook takes a few random ingredients  and randomly mixes them together.  

Maybe Tomato, Chicken, Rice  and half an onion as entree,  

Marshmallow, Pepper, Strawberries  and a quarter banana as a dessert.

Or Cucumber, Beef, Potatoes and two carrots,  

and blueberries, chocolate and  cream with a pinch of cinnamon.

Even with slight variation  and with only 100 ingredients,  

there are billions of possible recipes.  And likewise, with just a small selection  

of gene fragments, your cells create billions  of receptors. The details of this are so cool  

that they should get their own video – or  their own chapter in a book. In any case,  

by mixing up gene fragments, you get up to ten  billion different combinations. So in the end,  

you get billions of immune cells, and each of  them has one specific and unique receptor – the  

dish from our metaphor – that is able to  recognize one specific protein puzzle piece.  

In total, you end up with at least one cell  for every enemy that could possibly exist.

But here we run into a pretty dangerous problem  – if your adaptive immune system is making  

weapons that can attack every possible  protein puzzle piece in the universe…  

wouldn’t it also make some that can attack  your own cells? Yes, it happens all the time.

This is so fundamentally dangerous to  your survival that you have a whole  

organ that does nothing but work on preventing  this: The Murder University of your Thymus.  

Your Thymus is a chicken wing sized organ above  your heart and you've probably never heard of  

it. Interestingly, your thymus is one of the  reasons why your immune system weakens as you age,  

because it is in a constant state of decline once  you reach puberty. But what does the Thymus do?

In your murder university, your immune  system is putting your adaptive immune cells  

through an intense and deadly curriculum.  Basically it is showing them all sorts of  

protein puzzle pieces that are used by  your own cells to see how they react.  

When a young cell recognizes a body puzzle piece  and wants to attack it, the teacher cells order  

them to kill themselves and they are eaten up  and recycled. The immune system is so particular  

about this process that around 98% of your  adaptive immune cells that enter murder university  

die there. 2% graduate and get to do  their job of protecting you for real.

If this process goes wrong and cells escape that  can recognize your own protein puzzle pieces,  

this can lead to autoimmune disease, where  your immune system attacks your own body  

from the inside. Andt this, again  is another story for another time.

Ok, let us summarize. Your immune system has  two parts, one that defends you right after  

birth and one that carries the largest  library of superweapons in the universe  

but needs to boot up first. To create billions  of different superweapons, your adaptive immune  

cells recombine a part of their genetic code to  create a breathtaking variety of attack weapons.  

Then they enter a murder university that only 2%  survive to make sure they do not attack you. And  

then you end up with billions of different cells,  that in total are able to protect you against  

every possible enemy in the universe.  Now wait a second. If this is all true,  

why do we get sick at all? Why was a new disease  like Covid-19 able to kill millions of people?

Well everything we just learned about is just a  tiny, tiny window into the amazing struggle for  

life and death that plays out every day inside  your body and there are so many amazing details  

and questions here: How does your body actually  find the right cell in time to protect you?  

How do your enemies fight back and  overcome your immune system anyway?  

And what about all the things  that did not fit in this video?

Well, today finally marks the release of “Immune  – A Journey Into the Mysterious System that Keeps  

You Alive”, written by Philipp Dettmer,  the founder and head writer of Kurzgesagt.  

First we had to push the release back  because of cargo trouble and then because so  

many of you pre-ordered that we didn’t have  enough copies for the original launch day!  

Thank you so, so much for that!

Immune tells the epic story of your  immune system and will forever change  

how you think about your own body, how  you experience being sick and healthy.  

The book is written to be as fun and easy to grasp  as Kurzgesagt videos but it is able to dive way  

deeper iinto its subject. So go on a journey  through the hidden microverse within yourself.  

Witness deadly wars between billions of invaders  and cells, learn how your immune system actually  

works and protects you from cuts, cancer and  Covid. Never before have we so urgently needed  

to learn about how immunity works. Immune is fun  and great to look at and it even smells good.

Follow the link in the  description to order it today.  

This is the end of a decade-long  personal journey working on this,  

thank you all so, so much for your support.  And thank you for watching. And reading.