What is the Immune System?
3.5 billion years ago, cells figured out how to gather resources by leeching off of other cells. 541 million years ago, multicellular life formed and developed systems to defend against intruders. While we can’t directly analyze ancient immune systems, we can study it via the tree of life. The farther separated two creatures are on the tree and still share the same trait, the older the trait must generally be.
Modern animals are the height of the immune system development which at its core is a tool to distinguish the other from the self. While distinguishing the other from the self is the core, it is not the goal. Its overall goal is to achieve homeostasis, what we call health.
Ultimately the battle to stay healthy is futile and will be lost in the end, but our immune system fights to keep us going a little bit longer. Our immune system can also go wrong and be corrupted. When tricked, it can help spread diseases, protect cancer cells, cause allergies, and even get confused and deceive the body itself is the enemy. When too enthusiastic, it can cause unpleasant systems while it’s doing its job and can even cause death.
What Is There to Defend?
Immune cells are tasked with protecting the ~40 trillion cells in your body. Most threats are stopped by the skin, but intruders can still enter via your mucus membranes: windpipe, lungs, eyelids, mouth, nose, stomach, intestines, reproductive traces, and bladder.
What Are Your Cells?
Cells are the smallest units of life that we can clearly identify. Generally, something is alive if it separates itself from the universe around it, has a metabolism (intakes outside resources and expels inside garbage), responds to stimuli, grows, and can make more of itself. Cells are not conscious, have free will, or feelings. They are essentially biological robots. Cells have various “organs” inside them called organelles. These organs are surrounded by millions of molecules. Half of which are water molecules and other half consists of 1k-10k different kinds of proteins.
Proteins are the most fundamental organic building blocks and can be used for basically everything, from sending signals, constructing simple wall/structures to complex micromachines. Proteins are made from chains of amino acids (organic building blocks that come in 20 different varieties). A typical protein has 50-2000 amino acids (30k is largest known). There are billions of billions of possible proteins, but only 1-million to 1-billion useful ones. Your cells know which proteins to make thanks to the instructions on your DNA.
About 1% of your DNA is made up of building manuals for proteins. The rest is responsible for regulating which proteins are built when and how and how many of them at which time. Instructions on DNA are converted into proteins in a two-step process: 1. Special proteins read DNA and covers it into messenger molecules call mRNA (the language DNA uses to communicate orders). 2. The mRNA is then taken from the nucleus to the proteus production center called the ribosome. Here, mRNA is read and translated into amino acids that are put together in the order they arrived.
In the world of proteins, shape is everything. Their shapes determine what they can and can’t do. A proteins shape is determined by the amino acids it has and their sequence. As a protein is built, the chain folds together into a specific shape. Proteins work by interacting with each other in very complex ways. They move and interact by wiggling really fast. Because of their scale they behave very different compared to a human scale and can in theory move up to 5m/s if they were not surrounded by other molecules. This motion is called Brownian motion and is why water is so important for cells and leads to biological pathways.
Biological pathways is a fancy word to describe a series of interactions between individual things that lead to a change in a cell. A proteins specific shape enable them to fit together or interact with other proteins in specific ways. Sequences of these interactions, called pathways, cause cells to do things. An individual cell is pretty dump, but many cells working are pretty smart. The phenomenon occurs in many places in nature and is called emergence. It is the observation that the whole has properties and abilities that the parts do not have.
The Empires and Kingdoms of the Immune System
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 super weapons. But while it is powerful, one of its most important jobs is to make the Innate Immune System even stronger.