I know a bunch of people who claim that they are allergic to various foods, wheat being the most common. I would guess that most of them are victims of their own imaginations. But, my wife actually is deathly allergic to fish and seafood. Her throat swells up and she stops breathing and she just about died. That is a true allergic reaction.
I ran into somebody like this who had a bad reaction at work from eating at a potluck party. Really dumb - you don’t know what has touched what on somebody’s kitchen counter, etc.