Well, no one really knows why it happens. perhaps over-exposure and a flaw in your immune system finally breaks and the allergy appears? But there isn't any cut and dried reason why this happens. It's simply a breakdown in your immune system that allows the body to react to a substance that should not normally effect you.
Well here is what I found to be the case. I believe the problem starts in your intestine area. About 65-70% of your immune system is located in the intestinal track. If a break down begins to occur in the lining of the gut where small holes develop leaving openings directly into the bloodstream and undigested food particles begin to enter the bloodstream, your body begins to look at the undigested particles as dangerous invaders and is alerted by the brain to attack them. It doesn't matter if the food or medicine is an allergen or not ..healthy food or not. All it knows is the food has not been properly broken down making it dangerous instead of nutritious to the body so it must be dealt with. This is how inflammation can start to become prevalent in the body. It can start out as a small problem but can be the start of a slow breakdown of the entire immune system and you know what that can mean to the body as a whole over time as things continue to progress this way. I took my blood test through ThinkRite systems and not only live without allergy symptoms now but also learned more about what I just spoke about in greater detail there. I just cant say enough on how what I have learned has changed the way I look at allergies and what causes them....I wish you the best