From Nature:
Allergic reactions to metal are on the rise, although no one knows why. Now research suggests that bacterial infections experienced while wearing metal objects may be responsible. How an allergy to metal develops is a matter of much speculation. Yasuo Endo at Tohoku University in Sendai, Japan, and his colleagues had noticed that in previous animal experiments looking at metal allergies, researchers often used a chemical trigger or ‘adjuvant’ to encourage an allergy to form. In mice, researchers used hydrogen peroxide to stimulate the animals’ immune system and encourage a reaction. Endo and his team suspected that something was similarly provoking allergic reactions to form in humans — but not hydrogen peroxide, as we rarely come into contact with it. Instead, they focused their attention on a set of molecules called lipopolysaccharides, commonly found in bacteria, which are known to be able to provoke other immune responses.
The team injected small groups of mice with a nickel salt solution, with some groups also receiving a dose of lipopolysaccharides. Ten days later, the mice were injected with the same nickel salt in the ear, and the team measured the resulting inflammation. Mice that didn’t get the lipopolysaccharide dose in the first injection had almost no reaction to the nickel within a day of the second exposure; but mice that did receive lipopolysaccharides had an almost immediate, strong reaction to the metal.
More here.