You are Immune
You’re not a person but a planet, made up of roughly more than 40 trillion cells.
If your cells were human-sized, you would be as big as 20 Mount Everests, there is so much of you. For your creepy inhabitants, this makes your body an ecosystem, rich in space, warmth and resources. 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. 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 in 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. So, why are we not all killed by some new bacteria or virus? In a nutshell, you 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 soldier.
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 superweapon cells inside you to fight the black death, the coronavirus, 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 friends 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 can connect with a specific protein puzzle piece. So your cells have tiny puzzle pieces on their outsides that can click together, or recognize other protein puzzle pieces. When a cell connects with a protein and recognizes it as an “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 can 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 genetic code to create this stunning variety of receptors. The details are way too complicated for this article but in a nutshell, your adaptive immune cells have official permission to take a tiny part of their 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 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. Maybe Tomato, Chicken, Rice, 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 video — or their 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 can recognize one specific protein puzzle piece. In total, you end up with at least one cell for every enemy that could 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 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. It is showing them all sorts of protein puzzle pieces that are used by your 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 protein puzzle pieces, this can lead to autoimmune disease, where your immune system attacks your own body from the inside. And 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 murdered 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 can 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 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 article?