Allergy is a noun that describes a level of human energy. It is hard to quantify the meaning of allergy using even the most advanced tools medical science has to offer. It can best be described as “between comatose and supernova-like” on a scale that includes deceased, comatose and supernova-like.”
That, of course, leaves a lot of scope on either side of allergy. Another way to define it is that allergy is the level you have when you respond with “meh” when some one asks, “how are you.” Because the default answer tends to be “fine” even if you’re on your deathbed, it would be correct that allergy is a level of energy that is less than OK, although it is probably not life-threateningly low.