How the body attacks itself in autoimmune diseases

HostIt's pretty amazing how our bodies can fight off a cold without us even thinking about it. But sometimes that internal army gets confused and starts attacking the very thing it's supposed to protect. What goes wrong in our blood that makes it turn on us like that?

GuestTo understand that, we have to look at how the body trains its soldiers. Most of the guard cells in your blood are born in your bone marrow, but then they go to a sort of school in a small gland in your chest called the thymus. Think of this like a boot camp where the cells learn the difference between you and everything else. They're shown bits of every kind of tissue in your body, like your heart, your lungs, and your skin. If a young guard cell reacts too strongly to one of those bits, it fails the test. The body sees that this cell is a threat to the home team, so it pulls it aside and destroys it before it can ever go out into the blood. It's a very strict school. In fact, most of the cells that start the training end up failing and getting wiped out. Only the ones that can ignore your own parts while staying ready to fight germs are allowed to graduate and patrol your body.

HostIf the school is that strict and most of the cells get wiped out for failing, how do we ever end up with a self-attack?

GuestWell, no system is perfect. Sometimes a few of those aggressive cells slip through the net. They get out into the blood and wait. But even then, they usually don't start a fight right away. They need a spark to set them off. A big one is what we call mistaken identity. Imagine a germ enters your body that looks a lot like one of your own parts. A common example is the bug that causes strep throat. To your immune system, the surface of that bug looks very similar to the surface of the cells in a human heart. Your body builds up a huge army to kill the strep throat, but once the bug is gone, some of those soldiers see the heart and think the enemy is still there. They start attacking the heart valves because they can't tell the difference. The body is trying to do the right thing, but it's working off a bad set of photos.

HostBut our bodies have been dealing with strep throat for thousands of years. It seems like a pretty big flaw if our heart is basically a twin of a common germ. Why hasn't this been fixed by now?

GuestThat's a fair point, but it's not just about the germs. It's also about the world we live in today. There's a big idea that we might actually be too clean for our own good. For most of human history, our immune systems were constantly busy. We had worms, we had dirt in our food, and we were always fighting off something. Today, we live in very clean homes and use soap that kills almost everything. Because the immune system doesn't have its usual enemies to fight, it gets bored and twitchy. It's like a high-powered guard dog that's locked in a small room with nothing to do. Eventually, it might start chewing on the sofa or the rug. In this case, the sofa is your joints or your nerves. We see much more of this self-attack in wealthy, clean countries than we do in places where people still live close to the dirt and deal with regular bugs.

HostSo you're saying that if we all just stopped washing our hands, these diseases would go away? That feels a bit hard to believe.

GuestNo, it's definitely not that simple. You can't just go eat some dirt and fix it once the fire has already started. This is more about how the system develops when we're very young. And even then, dirt is only one piece of the puzzle. You also have to have the right genes for it. Think of it like a forest fire. To get a fire, you need two things. First, you need a pile of dry wood that's ready to burn. That's your genetics. Some people are born with a lot of dry wood, and others have wood that's damp and hard to light. But even if you have a huge pile of dry wood, you still need a match to start the flame. That match could be a bad virus, a huge amount of stress, or even some of the chemicals we breathe every day. If you have the genes but no match, you're fine. If you have the match but your wood is damp, you're also fine. You need the match and the wood to meet at the wrong time.

HostAnd once that fire starts, it sounds like there's no easy way to blow it out because the body thinks it's doing its job.

GuestThat's the hardest part. Unlike a normal infection where the fight ends when the germs are dead, in these cases, the thing the body is attacking never goes away because it's you. The body stays in a state of constant war, causing long-term swelling and damage to things like the brain, the gut, or the skin. We're learning how to calm those soldiers down without putting the whole army to sleep, but it's a delicate balance. We have found that some people with these conditions have a different mix of tiny life forms in their gut, which might be another way the body gets its signals crossed. We're even looking at how certain fats in our food might make the guard cells more or less likely to lose their cool. The goal is to find a way to send those confused cells back to school so they can learn to leave the home team alone.

HostIt turns out that those strict lessons in the chest are the only thing keeping our internal army from turning the whole body into a battlefield.

GuestWe're still trying to figure out why some people can live with those aggressive cells for decades before a single stressful week finally pulls the trigger.

HostThe wall that protects us is only as strong as its ability to know who belongs on the other side.