Matthias Goebeler at the University of Giessen in Germany and his colleagues looked at cells from the blood vessels of people with a nickel allergy. The team found that these cells react to nickel when a group of receptors known as Toll-like receptors (TLRs) are present.
When the team "knocked out" the genes behind each of the 11 types of TLR, they identified TLR4 as nickel's target – the same TLR that bacteria bind to, sparking the body's immune response.
"This receptor was thought to only be relevant for bacteria," says Goebeler, who suspects that other allergens may act in the same way.
Nickel and bacteria bind to different parts of the receptor,the team say, so blocking nickel's binding site could prevent allergic responses to the metal without stopping the body's inflammation response to infection.