How is it possible to be allergic to everything all of a sudden?

At the age of 13, I became allergic to most environmental things. Latex paint fumes (but not the touch), pollen, smoke, grass, and the list goes on. I asked the very question you pose today.

The body can store many irritants that cause no reaction for years. Suddenly the body rejects the subtance and a negative reaction occurs. It could be that simple itchy eyes and runny nose or the most severe anaphylaxis (closing of the throat) reactions.  Complex biochemical reactions can be inherited or caused by simply overloading the body with whatever you are suddenly finding as something to avoid. A cure is harder to find than symptomatic relief. I tried the shots (being a pincushion) for 12 years. I was able to get off the stronger antihistamines and even reduced my use of the inhaler that I needed almost daily at the height of my affliction. While I now can say that my allergies are under control, there are those who do not suffer at all. I am just not one of those lucky few.  Moving to a new place can seemingly  make your allergies disappear. Most likely your body will do what it did before: Collect and store the irritants until one day your body rejects  a single molecule more.  Find an antihistamine that gives you relief. If you do not, you will find you will catch more and more colds and viruses due simply to overload. Keep you hand off of your face (easier said than done). Try the shots, they work better for some than others in reducing your body's sensitivity to a particular allergen. Again, I stress symptomatic relief as the road to take. The "cure" does not exist to the best of my knowledge.