How come we have allergic reactions?

Complete ans unknown: It technically is misguided immune reaction. The body misinterprets a harmless protein or complex carbohydrate as a potential invader. Specifically it mounts a response as if there is a parasite in the area involved: skin, airway, gut - but if large areas involved or the trigger gets into the bloodstream (like bee sting or food allergy) life-threatening generalized allergic reaction=anaphylaxis.