Nat. Immunol. 17, 471 (2016); published online 19 April 2016; corrected after print 18 May 2017

In the version of this Correspondence initially published online, the following two relevant references were not included: Sporri, R. & Reis e Sousa, C. Eur. J. Immunol. 32, 3161–3170 (2002); and Montaudouin, C. et al. PLoS One 5, e10238 (2010). They have now been included, along with text explaining their relevance. The error has been corrected in the PDF and HTML versions of this article.