Some adverse reactions to foods are indeed allergic reactions, in the sense that they are an
Immunoglobulin E-mediated autoimmune response. Severe life-threatening peanut allergy, the kind of thing where you need an Epipen handy at all times, is one of the more commonly-encountered ones.
However, other adverse reactions to foods may
not fall into the category of true allergy, but rather chemical sensitivity. For various reasons not fully understood, cumulative high dietary intake of
salicylates and/or amines can trigger headaches, upper-respiratory irritation and other problems in susceptible individuals. Foods which contain high amounts of tyramine (one of the amines) include chocolate, cheese, red wine, processed meats (the classic
migraine triggers).
It is very frustrating to have to explain these adverse reactions to the lucky people who don't get them. Breaking out in a rash from exposure to poison ivy isn't hard to understand; breaking out in a rash from eating foods generally considered healthy causes disbelief. But it's just a fact that naturally-occurring substances in foods can and do cause harmful reactions in some. I have a bad time, for instance, with raw carrot: it makes the soft tissues in my nose and mouth swell up, and triggers a massive sneezing fit. Stone fruits like cherries, peaches and nectarines are all right if I eat them sparingly, but too much of them and my throat starts to close up.
It isn't hard for me to "get" that some kinds of animal muscle tissue, or other animal proteins, might not be well-tolerated by some individuals.