This article has been retracted at the request of the editor as the authors have plagiarized part of a paper that had already appeared in Nat. Rev. Immunol., 4 (2004) 941–952, doi:10.1038/nri1498. One of the conditions of submission of a paper for publication is that authors declare explicitly that their work is original and has not appeared in a publication elsewhere. Re-use of any data should be appropriately cited. As such this article represents a severe abuse of the scientific publishing system. The scientific community takes a very strong view on this matter and apologies are offered to readers of the journal that this was not detected during the submission process.
