Competing interests: The BMJ has judged that there are no disqualifying financial ties to commercial companies. The author declares the following other interests: research funding from the Wellcome Trust and Borne, payment for acting as an external examiner for the University of Cambridge and the University of Leeds, and royalties received for contribution to Immunology 9th edition (Elsevier). Further details of The BMJ policy on financial interests are here: https://www.bmj.com/sites/default/files/attachments/resources/2016/03/16-current-bmj-education-coi-form.pdf. Provenance and peer review: Commissioned; not externally peer reviewed.