Nicholas 'Avrion' Mitchison discovered the transference of transplantation immunity by sensitised cells, thereby providing the key evidence relating transplantation immunity to hypersensitivity reactions of the ‘delayed’ type. He devised a method for revealing mixtures of cells of different genotypes in vivo and used it to be equal first in demonstrating that the ‘radiation recovery factor’ is a graft of living cells and not a humoral agent. He carried out the most exact quantitative analysis of tolerance hitherto attempted in a cellular system and proved inter alia that persistence of tolerance depends on persistence of antigen. Avrion has thus made notable and distinctive contributions to immunology.

Professor Avrion Mitchison FRS died on 28 December 2022.