The Medical Explanation For Allergies
Allergies are nothing more than an inappropriate response to something that is perfectly harmless.
Your body sees an ordinary substance as foreign and attacks it sending you into an allergic reaction of red, itchy eyes, a scratchy throat and trouble breathing. It is essentially an error in the body's immune system where it does anything it can to combat this "bad, foreign" substance.
But this substance is perfectly harmless. What's worse is now a new association is turned into an engrained pathway in your nervous system which will associate that substance as a foreign object from there on out. It is a substance/association error that is made and learned by your nervous system.
An example of this is when you eat a normally harmless food and it makes you sick afterwards. (eggs for example) You get sick and you know what happens (no details necessary here). Your body will associate the sickness with the food that you ate so that most likely even months to years down the road even the sight or smell of eggs will make you nauseous and even a little sick.
But eggs are not the problem. It's the incorrect association that your nervous system has made that's the problem.