I would hate to leave the house without the odd necklace hanging round my neck. But I happen to be fortunate. Millions of other people are not. That is because a lot of jewellery contains the silvery-white metal known as nickel, which can cause disagreeable skin conditions. If nickel were confined to jewellery, things would not be so bad but it is also frequently found in zips, coins and mobile telephones for instance. And who, in our society, can easily dispense with any one of these items? 65 million people in Europe suffer from nickel allergy; that is a large part of the population. Nickel ions are able to creep off a necklace or a coin – following sweat or rubbing for example – and sink through the first layers of skin where they will trigger off an immune response resulting in dermatitis. But why does it happen in some people and not in others? The answer seems to reside in a very small region of a protein known as the toll-like receptor 4, or TLR4, which has been shown to be at the heart of nickel allergy.