Nat. Immunol. 10, 840–847 (2009); published online 13 July 2009; corrected after print 19 August 2009

In the version of this article initially published, citation of the linked articles published in the same issue (ni.1769 and ni.1766) is missing. These should be cited on page 841 at the end of the first paragraph of the Results section. The error has been corrected in the HTML and PDF versions of the article.