What is an autoimmune disease? And why is that an article on Microbial Mondays?
Updated: Aug 26, 2020
If you are a reader who doesn't know me well, you might not know that I am an academic and study infection and immunity. Today, I was reading a scientific article about Juvenile Idiopathic Arthritis which is a type of autoimmune disease that, among other things, basically results in arthritis in kids. After reading an article, I always like to sit in silence for a while and ponder the messages within it. After this one, my mind went wandering through the world of autoimmune diseases – and what a huge world that is! It's pretty likely that you actually know somebody who has an autoimmune disease – for instance heart disease, arthritis, and lupus are fairly well-known examples. But these diseases have all sorts of different symptoms: Some affect muscles, some affect joints, some affect the skin, and some affect the whole body. What is it that binds this wide variety of diseases together? And why would I write about this on a blog called Microbial Mondays? Let's find out.
In a nutshell, autoimmune diseases happen when your own immune system gets confused. Let's think of the cells of your immune system like an army protecting your body from infections. When the soldiers in this army go through training, they learn to recognize different markers on the other cells of your body. By displaying these markers, the cells are basically displaying the flag of a friendly nation to the immunity-army. This tells the army, "DON'T ATTACK!' There are pretty much gazillions of different friendly flags within your body, but your immunity-army (immunitarmy? Yes, let's call it that from here on out) is smart and usually doesn't attack anything with these friendly flags.
Normally, these tiny soldiers roam around looking for markers that they've never seen before. We have an over-protective army, in that if it sees a new flag that it didn't see during training, it attacks. For instance, viruses and bacteria that cause disease have new flags, and become targets of the army.
Unfortunately, immunitarmy sometimes doesn't properly train its recruits. Sometimes, when a new soldier is being trained, it will mistakenly recognize a friendly flag as unfriendly. This means that when this soldier encounters that particular friendly flag, it will attack it – and then divide into a battalion that searches for more copies of that friendly flag and continue attacking.
This is the basis of an autoimmune disease: your own soldiers have started attacking you. But we're still missing one key piece of information: what goes wrong in the training of these misinformed soldiers?
In the last decade or so researchers have discovered that certain bacteria and viruses can interfere in the training of our immune cells! One way they can do this is via a trick called mimicry. You might have heard this word before, in the context of a non-poisonous snake looking almost identical as a poisonous one, or a butterfly's wings looking like a scary animal to its predators. Mimicry is basically being a copy-cat, and is seen all over the realms of biology.
The type of mimicry that microbes use to confuse our immunitarmy is called 'molecular mimicry'. Microbes mimic our tiny, molecular friendly flags! This is good for the microbes, because if they display these friendly flags, they aren't attacked, or at least aren't attacked with as much gusto, by our immunitarmy. But, here we must add a layer of complexity: Microbes display many different flags all at the same time, so it is possible that these copy-cat microbes can give a mixed message of both friendly and unfriendly flags. Sometimes, the combined message can indeed lead to a full-on attack by our immunitarmy. Battalions are formed, and some of those battalions are also directed against the mimicked friendly flag. The good news is that, if this happens, the infection will be cleared up. The bad news is that the newly formed battalion can go on to attack other cells in your body which display the original friendly flag. And this is what we call an autoimmune disease.
I do have to give a bit of a disclaimer at the end here. This is a very simplified version of what happens in the development of an autoimmune disorder, and I left out a lot of details to make it as clear as possible. In addition, the link between infection with certain microbes and development of autoimmune disease is not there for all autoimmune diseases. However, this is a growing field of research and we can all expect to hear more about it in the next years!
Until next week – stay healthy and happy!