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MLA Full: "Can You Be Allergic To Sunlight?" YouTube, uploaded by SciShow, 17 June 2017,
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For you, going outside might mean a sunburn if you're out there too long without enough sunscreen, but for some people, any sun exposure can lead to hives and rashes within minutes!

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We all know that the sun can be dangerous.

The UV radiation that beams down on you at the beach can damage your DNA, giving you painful burns and eventually, possibly cancer. That should be reason enough to wear sunscreen.

But for some people, sunlight means hives or itchy rashes. That’s right, you can be allergic to the sun. If you have a condition called solar urticaria, big, itchy welts develop within minutes of sun exposure.

These hives die down within a day, but as you can imagine, this disease seriously puts a damper on going outdoors. Scientists don’t fully understand why this happens, but they think it’s pretty similar to how your immune system might overreact to peanuts if you have a food allergy, or to pollen if you have seasonal allergies. In those cases, the immune system sees allergens like peanut or pollen proteins as foreign invaders, and arms special cells with antibodies that recognize them.

So when those proteins pop up again, those immune cells spring into action and release a bunch of histamine. Histamine is a compound that spurs blood vessels to become leaky and muscles to contract, which helps more immune cells flood in and fight off any junk, but also causes the telltale symptoms of an allergic reaction, like red blotchiness or swelling. In solar urticaria, that same process seems to happen, but the molecule that’s causing the immune system freak out is a mystery.

The leading theory is that some people have light-absorbing molecules in their skin that transform into allergens when the right wavelength hits them. In severe cases, people can even develop welts from lightbulbs or when they’re wearing thin clothing outdoors, or even go into anaphylaxis. The good news is that this sun allergy is extremely rare.

Of all the people who get hives, only about half a percent of them do because of the sun. But there’s a similar condition that affects as many as 10 to 20 percent of some human populations called polymorphous light eruption, or PMLE. It’s also a sun-triggered rash.

But, as the name “polymorphous” suggests, it can be a bit of everything -- from itchy, fluid-filled blisters to simple raised bumps or red patches. Instead of showing up almost immediately, like solar urticaria, it can take hours or even days, and it sticks around for awhile too. How the immune system goes overboard in this case is even less well understood.

But because of the delayed timing, it’s thought to be sort of similar to a poison ivy reaction, which activates immune cells called helper and killer T cells and causes inflammation. Except instead of an oily sap, the trigger in PMLE is another mysterious light-sensitive molecule. Fortunately, PMLE doesn’t cause scarring, and with repeated exposure to the sun over time, the rash outbreaks often improve.

So, while there may not be sunlight-fearing vampires roaming the planet, there are some people who need to live a little bit like them. Thanks for asking, and thanks to all of our Patreon patrons who keep these answers coming! If you want to learn more about allergies, we have a video where Michael goes deeper into the science of these immune system mishaps.