The number of cells that make up our body are hundreds and hundreds of billions, to which we can add a few more billion units of viruses and bacteria. This immense apparatus is kept under control by a formidable and elegant self-defence organ: the immune system. It repairs the tissues, eliminates toxins, but above all it fights the dangerous intruders known as pathogenic microorganisms. These are basically disease-causing agents, and are mainly divided into three groups: parasites, viruses, and bacteria. In their deadliest form, we can imagine them as little killers. Little because they are in fact microscopic: a single human cell can contain up to a few thousand bacteria and, in terms of proportion, we could actually insert a few thousand viruses into a single bacterium. Truly amazing numbers.
However, there is also an important fact that must be clarified: not all viruses and bacteria are pathogenic and enemies of our body, indeed, if anything, the opposite is true: only about one percent of bacteria cause disease, and some viruses are actually essential for our survival. Think of the bacterial flora in our stomach which facilitate the digestive process.
Obviously, without an immune system we would not be able to fight disease, our bodies would not even be able to recover from the smallest accident, so it goes without saying that we would not be able to exist. So how does the body actually fight unwanted invaders? How does the incredible and fascinating machine that is the immune system work? How have we managed to survive for so many centuries, skirting all the dangers that the world has put us in? The simplest, clearest, and most interesting way to answer this question is to take a look at one particular aspect in the development of the immune system, and more specifically, at how we have gradually come to learn about it and discover how it works.
