Nickel allergy in relationship to previous oral and cutaneous nickel contact.

Potential relationships between the development of nickel allergy and previous ear piercing or orthodontic treatment with nickel-containing appliances were studied in 294 patients. We found 77 (31.2%) of 247 patients with pierced ears were allergic to nickel compared to only three (6.4%) patients without pierced ears (p = 0.001), which confirms earlier suggestions that nickel allergy (as assessed by patch testing) is promoted by ear piercing. If orthodontic treatment preceded the event of ear piercing, the frequency of nickel allergy was reduced from 36% to 25%. This supports the view that oral allergenic contacts may induce immunological tolerance.