eli5 – How does our skin know to react to an allergen on the surface?
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Our skin doesn’t block out everything. Because the skin needs to breathe and move, it can’t be 100% everything-proof. Some particles are very small, so they get *into* our skin. Normally this is fine, because one of the jobs your skin has is preventing stuff from getting into your body, so as long as these pollutants don’t get *past* your skin, you’re fine.
But when you’re allergic to something, your body sees that as a much more dangerous threat than it is. Your immune system gets really fired up about something small. So, say, you’re allergic to nickel, and a lot of cheap jewelry is made out of nickel. Then you put a nickel ring on, some nickel gets into your skin, you have lots of tiny little blood vessels in your skin that carries that nickel into the body, and the body has a huge temper tantrum. The immune system goes absolutely wild, saying “there’s a threat on your finger! Neutralize it! Get it in any way we can!”
*How* it tries to neutralize the threat depends on the person. You can get a hot, swollen finger, a rash on the skin, blisters or hives. It’s all your own body trying very hard to get rid of a dangerous intruder.
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