Question: Why are people allergic to things, what causes allergies?

Matthew Pankhurst answered on 26 Jun 2013:

As someone that suffers from hayfever I know this one! Allergies are when your body overacts to something that it see as a threat. All the time your body is battling invaders like germs, bugs, chemicals and even light. The way we deal with this is detecting them and then getting rid of them (except light – we just keep repairing our skin and may get a tan). Lets say a fly goes up your nose – you sneeze it out! That’s an easy, big example. But what about dust and other tiny particles that could be carrying germs or just generally clogging things up? When our skin or special detecters in our eyes, ears, nose and throat sound the alarm the body reacts by producing mucus which flushes them out again. Mucus is wonderful stuff – http://en.wikipedia.org/wiki/Mucus
All this activity to produce mucus causes the areas around where you make mucus to swell up a bit with blood, because having a good blood supply is needed to do pretty much anything in your body.
When people are allergic their body over-reacts to whatever caused the problem, this can be due to how we recognise different chemicals or dust or pollen or whatever it may be. How we recognise things is down to our immune system, which is largely controlled by our genetics, but also what we were exposed to as children. This is an ongoing field of research, because we don’t understand exactly why individual people are allergice to certain things and other aren’t…