The reason why allergies are caused by nickel has been finally discovered by researchers.
Researchers at University of Giessen, Germany found that the metal directly activates a member of the family of receptors that act as gatekeepers of innate immunity.
According to Nature, activating this receptor, called Toll-like receptor 4 (TLR4), generates a 'danger signal' that promotes inflammation - causing itching, tenderness, swelling and rashes on the skin - and recruits other immune cells such as T cells to the area.
The study also suggested that nickel itself was acting as the signal for T cell recruitment.
They identified two regions in the human protein that contain an amino acid called histidine - not present at those positions in mouse TLR4 - to which nickel might bind and so trigger the signalling cascade.
The study not only solves the long-standing mystery of how nickel causes an allergic reaction, but also "opens up another avenue of thought" about how Toll-like receptors work, says Anthony Gaspari, a dermatologist at the University of Maryland in Baltimore.
The study is published in Nature Immunology.