If you never heard of the Bathtub Effect for Allergies, I may have made it up. That doesn’t make it untrue. Here’s the issue. People have allergic symptoms and then go to an allergist to get tested. They get tested for bunches of possible allergies and come up negative on all of them. These tests typically don’t show minor allergic reactions, and I believe most of us are somewhat allergic to many things. Moreover, I believe that these compound.
So, like Archemedies in his bathtub, when you displace enough water, it overflows. Which is to say that if you have enough minor allergic reactions, they can add up to one big one. The trick then is to eliminate as many as you can, see how you feel, and then gradually add back in until you have symptoms. Or, better yet, don’t.