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 research charity Borne; payments to act as an external examiner for the University of Cambridge and the University of Leeds; and royalties received for my contribution to Immunology 9th edition (Elsevier). Further details of The BMJ policy on financial interests is 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.