Why does nickel bring some people out in a rash?

THAT'S THE WHY: When we pick out jewellery, we put some thought into it: is the colour, shape and size right for me? Is it too blingy or maybe it's not flashy enough?

But for some people, there's another burning question: Does it contain nickel?

Nickel is one of the most common triggers of contact hypersensitivity in the industrialised world. Affected people can develop skin bumps or rashes after direct contact with the metal, whether it's from wearing consumer items like jewellery or clothing (where clasps, buckles, buttons or zips may contain the metal) or through occupational exposure.

But why does a person react? Scientists in Germany have figured out a key step in the process: in a paper published online in Nature Immunology this month they showed that nickel triggered an immune response in humans by activating a molecule called Toll-like receptor 4 (TLR-4).

Toll-like receptors are a family of molecules in the body that act like ignitions for our innate immune system - the presence of a pathogen like a bacterium can turn the key and that triggers an immune response.

The German study showed that nickel activates TLR-4 in humans, but not in mice, and it pinpointed the specific portion in human TLR-4 that seems to mediate the immune response to the metal.

While it's an early finding, the authors note that it opens up new possibilities to look at blocking that particular response to nickel.