Extreme polymorphism at MHC loci is generally thought to be maintained by natural selection at a population level. Because allelic MHC molecules differ in their influence on the immune response, polymorphism ensures that the response repertoire is broad and that the group survives microbial challenge. Here Leif Anderson and his colleagues propose a different view, based on the observation that certain pathogens passively acquire MHC antigens from their primary host and carry these antigens into subsequent hosts. An immune response to the pathogen may depend on recognition of the non-self MHC antigens.
