John Joseph Thomas Hayes Owen. 7 January 1934 — 5 January 2023

John Owen was an anatomist and immunologist who conducted pioneering work on the origins and development of the two major classes of lymphocytes, the cells that are the central players in the adaptive immune systems of vertebrates. He was a meticulous biological experimenter who liked to keep questions simple and precise. His observations contributed to two key discoveries: that mouse and avian stem cells migrate into the embryonic thymus and give rise to T lymphocytes; and that in birds the bursa of Fabricius is the tissue in which stem cells develop into B lymphocytes. He also helped to establish that in mammals the earliest B lymphocytes develop in the liver of the fetus. He and his colleagues went on to define how combinations of several cell populations in the thymus provide the settings in which mammals first generate and then select from the variety of T lymphocyte lineages and immune specificities that will equip them with healthy immune surveillance and responses. His wry Liverpudlian sense of humour let him engage easily with everyone, from any walk of life, and he was an inspiration to young colleagues, many of whom would go on to flourish in the research areas that he pioneered. John died on 5 January 2023, from prostate cancer and other ailments: but he was in the gym only a couple of weeks before he died. He is buried in Kirkdale cemetery, Lancashire, under a family tombstone that also celebrates his grandfather, his father and his mother: his epitaph reads ‘One push-up too many’.