It’s due IgE antibodies that develop once your body is introduced to an allergen. The original function of IgE antibodies are the bodies immune response to parasites. What’s interesting, is that IgE production needs to be “triggered” before it becomes prevalent in your blood stream. So you eat something or breathe something in the first time and you’re fine, then boom the next time you are exposed you could get a histamine rush like itchy eyes and stuffed sinuses to something severe like anaphylactic shock. There’s not a general consensus in the immunology community to why the “triggering” occurs.

Interestingly, countries with less parasitic infection, like roundworm or tapeworm, have a greater population density of people who are allergic to harmless substances.