Also, the obligatory disclaimer: I'm not an immunologist, but a science communicator and immune system enthusiast. This book will not make every immunologist happy—what became obvious right from the start of the research is that there are a lot of different ideas and concepts about the details of the immune system and there is a lot of disagreement between the scientists holding these ideas. (Which is how science is supposed to work!) For example, some immunologists consider certain cells useless fossils, while others think they are crucial for your defenses. So as much as possible this book is based on conversations with scientists, the current literature that is used to teach immunology, and peer-reviewed papers.
Still, at some point in the future, parts of this book will need an update. Which is a good thing! The science of immunology is a dynamic field where a lot of amazing things are happening and different theories and ideas are in flux with each other. The immune system is a living topic where great discoveries are still happening. Which is great, because it means we are learning more about ourselves and the world we live in.
OK! Before we jump in and explore what your immune system is doing, let us define the premise first, so we have solid ground to stand on. What is the immune system, what is the context it works in, and what are the tiny parts that do the actual work? After we have covered these basics we will explore what happens if you hurt yourself and how your immune system rushes in to defend you. Then we'll explore your most vulnerable parts and see how your body scrambles to protect from a serious infection. And lastly, we'll take a look at different immune disorders like allergies and autoimmune disease and discuss how you can boost your immune system. But now let us get to the very beginning of this story.
MEET YOUR IMMUNE SYSTEM
WHAT IS THE IMMUNE SYSTEM?
The story of the immune system begins with the story of life itself, almost 3.5 billion years ago, in some strange puddle on a hostile and vastly empty planet. We don't know what these first living beings did, or what their deal was, but we know they very soon started to be mean to each other. If you think life is hard because you need to get up early in the morning to get your kids ready for the day, or because your burger is only lukewarm, the first living cells on earth would like a word with you. As they figured out how to transform the chemistry around them into stuff they could use while also acquiring the energy needed to keep going, some of the first cells took a shortcut. Why bother with doing all the work yourself if you could just steal from someone else? Now, there were a number of different ways to do that, like swallowing someone else whole, or ripping holes into them and slurping out their insides. But this could be dangerous, and instead of getting a free meal, you could end up as the meal of your intended victim, especially if they were bigger and stronger than you. So another way to get the prize with less of the risk might be to just get inside them and make yourself comfortable. Eat what they eat and be protected by their warm embrace. Kind of beautiful, if it wasn't so horrible to the host.
As it became a valid strategy to become good at leeching from others, it became an evolutionary necessity to be able to defend yourself against the leeches. And so microorganisms competed and fought each other with the weapons of equals for the next 2.9 billion years. If you had a time machine and went back to marvel at the wonders of this competition, you would be pretty bored, as there was nothing big enough to see other than a few faint films of bacteria on some wet rocks. Earth was a pretty dull place for the first few billion years. Until life made, arguably, the single largest jump in complexity in its history.
We don't know what exactly started the shift from single cells that were mostly on their own to huge collectives working closely together and specializing.*
* Although funnily enough, it may actually have been a side effect of single-celled organisms being mean to each other. As at one point one cell swallowed another but did not devour it. Instead these two cells started arguably the most successful partnership on planet Earth that is still going strong today. The "inside cell" (that we know as "mitochondria" today) specialized in making energy available for the host, while the "outside cell" offered protection and delivered free food. This deal worked very well and enabled the new super cell to grow in complexity and become more and more sophisticated.