I've talked about MHC before. ¹ It is important because it has a key role in the adaptive immune system and is illustrative of an important dynamic in evolutionary genetics, balancing selection, which perpetuations extreme polymoprhism within populations. Over time a functionally constrained locus which has an important fitness effect should fix toward the most advantageous allele. Polymorphism, where the modal allele is exhibited at lower than 95% frequency, suggests a population in transition. One can imagine such a scenario in a newly admixed population which has not had time to fix in populations. But MHC is different, many of the alleles on this locus persistent across species and have extremely deep evolutionary roots. There are two standard reasons given for this, a) heterozygote advantage or b) frequency dependent selection. No matter the details of the case, the importance of MHC and the persistence of polymorphism across many lineages and deep back into time is one of those truisms that thankfully takes a little of the territorial sloppiness that is habitual in much of biology. But assumptions need to be tested, so I pointed you to this paper, MHC class I genes in the tuatara (Sphenodon spp.): Evolution of the MHC in an ancient reptilian order). Here is the interesting part:

Preliminary analysis of variation among individuals from an island population of tuatara indicates these loci are highly polymorphic....

The Tuatara is an extremely ancient reptilian species on a lonely branch of that class of animals.

1 - MHC also might play a role in inbreeding and outbreeding avoidance.