Imagine you were the grand architect of the immune system. Your job is to organize the defenses against millions of intruders that want to take it over. You get to build whatever defenses you like, although the accountants remind you that the body is on a tight energy budget, has no resources to spare, and they kindly ask you to not be wasteful. How would you approach this monumental task? What kind of forces would you put at the front and which ones would you hold in reserve? How would you make sure that you could react strongly to a sudden invasion but also prevent your army from exhausting itself too quickly? How would you deal with the massive scope of the body and the millions of different enemies you would have to account for? Luckily, your immune system has found many beautiful and elegant solutions for these problems.
As we alluded to in the last chapter, the immune system is not a singular thing but many different things. Hundreds of tiny organs and a few bigger ones, a network of vessels and tissues, billions of cells with dozens of specializations and quintillions of free-floating proteins.
All these parts form different and overlapping layers and systems, so it’s helpful to imagine them as empires and kingdoms that, in unison, defend the continent that is your body. We can organize them into two very different realms that together represent the most powerful and ingenious principles that nature found to defend your continent of flesh: The Realm of your Innate Immune System and the Realm of your Adaptive Immune System.
The Realm of the Innate Immune System contains all the defenses you are born with and that can be employed mere seconds after an invasion occurs. These are the basic defenses that go back to the very first multicellular animals on earth and they are absolutely crucial for your survival. One of its most central features is that it is the sort of smart part of your immune system. It has the power to tell self from other. And once it detects other it immediately springs into action. However, its weapons are not tailored to identify any specific enemy, but instead they try to be effective across a wide range of common enemies. It doesn’t have specific weapons against specific types of E. coli bacteria, for example, but against bacteria in general. It’s designed to be as widely effective as possible. Think of it like your basic starter kit: it has all the fundamentals, not the specialized items you’d get with an advanced kit. But without the fundamentals, the specialized items are all but useless.
Without your Innate Immune System, you would be overwhelmed and killed by microorganisms within days or weeks. It does the heavy lifting and most of the actual fighting. The vast majority of your hundreds of billions of soldier and guard cells are part of your innate immune system. These are pretty rough fellows that prefer bashing in heads over talking and thinking. Most microorganisms that successfully invade you are killed by your innate immune system without you even noticing. Since the Innate Immune System is the first line of defense it is not just responsible for throwing soldiers at danger, it also has to make crucial decisions: How dangerous an invasion? What kind of enemy is attacking? And are more heavy weapons necessary?
These decisions are vital because they influence what sort of weapons your immune system as a whole will deploy. A bacterial invasion needs a very different response than a viral invasion. So while a fight is going on, the Innate Immune System gathers intel and data and then it makes the decisions that in many cases will decide your fate. If your innate immune system thinks an attack is serious enough, it has the power to activate and call the second line of defense to mobilize and join the fight.
The Realm of the Adaptive Immune System contains specialized super cells that coordinate and support your first line of defense. It contains factories that produce heavy protein weapons and special cells that hunt and kill infected body cells in the case of viral infections. Its defining feature is that it is specific. Unbelievably specific, in fact. Your Adaptive Immune System “knows” every possible intruder. Its name, what it had for breakfast, its favorite color, its most intimate hopes and dreams. The Adaptive Immune System has a specific answer for every single possible microorganism that exists on this planet right now—and for every single one that can evolve in the future. Think about how creepy that actually is. If you were a bacteria, for example, all you would want is to get into a human and find a place to make babies but suddenly there are agents that know your name, your face, your personal history, and all of your most intimate secrets and they are armed to the teeth.
This breathtakingly specific defense and how it works will be a focus of future chapters, but for now, just remember that your Adaptive Immune System possesses the largest library in the known universe, with an entry for every current and future possible enemy. But not only that, it also is able to remember everything about an enemy that showed up only once. It is the reason most diseases are only able to manifest themselves once in your life. But this knowledge and complexity come with downsides.
In contrast to the Innate Immune System, your Adaptive Immune System is not ready yet when you are born. It needs to be trained and refined over many years. It starts as a blank slate and then gets progressively more powerful, only to get weaker again as you age. A weak Adaptive Immune System is one of the main reasons young and old humans are often much more likely to die from diseases than people in the middle of their lives. Mothers actually lend their newborn babies a bit of their adaptive immunity in their mother’s milk to help them survive and give them some protection!
While it is easy to think of the Adaptive Immune System as your more sophisticated defense, one of the most important things it actually does is to make your innate defenses stronger by motivating your innate soldier cells to fight harder and more efficiently (but more on that later).
For now, let’s summarize: Your Immune System consists of two major realms: Innate and Adaptive Immunity. Your Innate Immune System is ready to fight after birth, and can identify if an enemy is not self, but other. It does the down-and-dirty hand-to-hand combat, but it also determines what broad category your enemies fall in and how dangerous they are. And finally it has the power to activate your second line of defense: Your Adaptive Immune System, which needs a few years before it is ready to deploy efficiently. It is specific and can draw from an incredibly large library to fight every possible individual enemy that nature can throw at it, with powerful superweapons. But while it is powerful, one of its most important jobs is to make the Innate Immune System even stronger.
Both of these realms are interconnected in a deep and stunningly complex way. And it is in the interactions between these two systems wherein lies some of the magic and beauty of your immune system.
To explore the different realms with the attention they deserve, the rest of this book is organized into three major parts. In part 2 we will experience an invasion that will occur through your skin and by bacteria and in part 3 we will witness a sneaky surprise attack on your mucosa by viruses. In part 4 we will then see how everything comes together and discuss specific disorders and diseases, from autoimmune to cancer.
In some other blog, we will see what happens if your borders are breached.