Non-Hair Allergy Question

This may be more of a non-direct hair question and thought you could answer this from your medical background. I’ve been consuming a bit of soy for hair protective measures and a friend of mine said that her roommate developed an allergy to soy because she got sick all the time. I always thought an allergy was manifested by itching, hives, etc. and not being sick. Does this make sense?

Bad reactions to natural substances do happen and it is not necessary to have itching, hives, etc. For example, some people get sick from dairy products and they do not develop itching, hives, etc but will develop ‘stomach’ problems like cramps or diarrhea.