Across jawed vertebrates, the genes of the major histocompatibility complex (MHC) make cell-surface proteins that display pathogen peptides to the immune system (Murphy & Weaver, 2017). The MHC is central to host-pathogen interactions, and different MHC alleles encode proteins that can differ in their effectiveness against particular pathogens (Sepil et al., 2013;Wegner et al., 2003). This makes MHC variation a key component of host-parasite dynamics