There's at least 4 different types of hypersentivity reactions that the immune system can have - it doesn't have to be an allergic reaction to be a hypersentivity.

It is a lot easier sometimes to claim an allergic reaction than explaining anything more... It's a lot easier to get people to respect my wishes that I don't want them to smoke around me if I say I have asthma or am allergic as opposed to just saying "I really don't deal well with cigarette smoke" 'cause for whatever reason I have a really difficult time breathing & will end up getting a chest cold if I am around smokers. of course, I've never bothered to go to the doctor to figure out why I don't deal well, so there may be an actual reason, however I've had a lot of people imply that I'm faking it or whatever.

My brother had a kidney disease for 2-3 years when we were young - he was basically on a no-salt to trace salt diet, which meant it was easier for my Mom to put us all on a low salt diet, no salt bread, no salt peanut butter, salt substitute in anything my Mom cooked. He had to test the salt levels in his urine everytime he went to the bathroom, and everything had to be logged. So there are medical conditions where you can't handle salt.