Skin. We all have it, so we usually take it for granted. But imagine, just for a moment, a life without skin – imagine living with your insides bare to the outside world. If you've ever had a burn, a nasty popped blister, or even a papercut that's become infected, you've had a little taste of that. But imagine not having this "physical barrier against infection", as an immunologist would call the skin, at all. Your life would be a lot harder. All day, every day, you'd have to take extra precautions to deter invading microbes from taking hold in your body. Your clothing would have to be sterile, for one, and Spartan or mud races definitely wouldn't be as popular as they are now.
This post is dedicated to skin, which is not only a physical barrier against infection, but also a fully-fledged immune organ in and of itself. The credit for the inspiration behind this post goes to Denis – thank you for answering my call for topics!
As you probably realized while pondering a life without skin, this outer layer of the human body is our first defense against invading microbes. The skin is actually a pretty complicated organ, and there are multiple layers to it. After the layer of dead cells on its very outside, the first layer of living cells of the skin is called the epidermis. And this layer has some pretty cool cells, which are named after a guy called Paul Langerhans.
This is what epidermis looks like on its own! This photo is of the first time I held a piece of donated epidermis in our lab – thus the scrunched nose and slightly grossed out expression.
Langerhans, a German, discovered these strange-looking cells while he was still an undergraduate student. After injecting gold into human skin, he saw cells that looked a bit like nerves, but didn't behave like them. They had long, stringy arms, and later other scientists also saw that they had spots shaped like tennis rackets. Quite a strange appearance indeed!
It turned out that these cells, which we now call Langerhans cells, are the "most peripheral outpost of the human immune system". In simpler terms, these cells are the very first ones to notice if something invades into the skin. They act as sentries, controlling the access to anything that wishes to travel deeper into the body. If you poke yourself with a needle with some bacteria on the tip, these cells will probably be the first to notice. And once they notice something invading, they spring into action!
If these sentry Langerhans cells come across an "antigen", which is more or less an immunologist's term for a molecule that has entered somewhere it doesn't belong, they will gobble it up. That's right, they eat it! But they don't stop there – then they wear it. Langerhans cells gobble up the costume worn by the invading bacteria or virus, and then they put it on themselves, to show the other cells of the immune system what the latest threat looks like. And we haven't even gotten to the coolest part about Langerhans cells yet.
My supervisor, Dr. Carla Ribeiro, recently found out that Langerhans cells are able to thwart HIV! Remember those weird tennis racket spots that Langerhans cells have? These cells are able to recognize HIV, send it to tennis racket-shaped prison, and then later actually break down the virus, which means that infected Langerhans cells don't pass on HIV to other cells in your body. These sentries can catch even some of the the sneakiest microbes!
I hope that I've convinced you that your skin is a pretty awesome immune organ, and that you should be thankful for it. On the more practical side of things, one of the easiest, and perhaps best, ways to keep yourself safe from infections is just to take care of your skin! That means, try not to pop your blisters, bite around your fingernails, or pick at scabs. Moisturize, and clean out and take care of cuts when you get them. Taking care of your skin means that pesky microbes will have a much harder time invading!
Until next week, keep your epidermis happy!