As a PhD student at Harvard in the late 1970s, Jim Kaufman identified and characterized class II molecules encoded in the human major histocompatibility complex (MHC). A model based on a chance observation led to his life-long interest in the evolution of immunity. At the Basel Institute for Immunology, he looked for MHC molecules and genes in many organisms. Eventually, he focused on the chicken, the only non-mammalian vertebrate for which there was extensive information on pathogens, vaccines, immune responses, genetics and development. The work continued at the Institute for Animal Health in Compton UK, where he showed the importance of genomic organisation and gene co-evolution in the evolution of the MHC. For the last ten years, he has been the Professor of Comparative Immunogenetics at the University of Cambridge, developing scenarios for the origin and evolution of the adaptive immune system of jawed vertebrates, refining our understanding of how chickens respond to infectious pathogens, and using the simpler chicken MHC to discover phenomena difficult to discern in mammals.