Content may be subject to copyright.This is a post-peer-review, pre-copyedit version of an article published in Nature Reviews Immunology. The final authent... ...This is a post-peer-review, pre-copyedit version of an article published in Nature Reviews Immunology. The final authenticated version is available online at: https://doi.org/10.1038/nri3401-c1. The following terms of use apply: https://www.springer.com/gp/open-access/publication-policies/aam-terms-of-use.