Allergic to EVERYTHING... what's causing it?

My friend is literally allergic to everything. He can't eat citrus fruits because the acid makes his throat close up, the tiniest bit of air pollution (fireplace smoke, something burning on the stove) makes him wheeze and cough for hours, and if you rub or scratch his skin, especially his torso or neck, it raises and gets red like a rash. He is constantly itching. The funny thing is, he says that he had a very good immune system as a child and it is just now weakening. Anybody have any ideas as to what this might be?