Page 320 - Veterinary Immunology, 10th Edition

MHC class Ia loci encode very polymorphic genes. For example,
VetBooks.ir the H-2K locus in the mouse codes for more than 100 alleles. Since

there can never be more than two alleles per locus in any individual
animal, it appears that this number of alleles has evolved to

maximize polymorphism. This may protect the population as a
whole from complete destruction. Because of MHC polymorphism,
most individuals in a population carry a unique set of class Ia
alleles, and each individual can therefore respond to a unique

mixture of antigens. When a new infectious disease strikes such a
population, it is likely that at least some individuals will possess
MHC molecules that can bind the new antigens and trigger
immunity. Those that can respond will mount an immune response

and live. Those that lack these molecules cannot respond and will
die.
When large populations of humans or mice are examined, no
single MHC haplotype predominates. In other words, no single

MHC haplotype confers major survival advantages on individual
animals. This reflects the futility of the host attempting to bind all
the antigens in a population of invading microorganisms. Microbes
will always be able to mutate and evade the immune response

faster than we mammals can develop resistance. Any changes in an
MHC allele may increase resistance to one organism but at the same
time decrease resistance to another. It is more advantageous
therefore for the members of a population to possess many highly

diverse MHC alleles so that any pathogen spreading through a
population will have to adapt anew to each individual.
Highly adaptable social animals, such as humans or mice, with
large populations through which disease can spread rapidly,

usually show extensive MHC polymorphism (Fig. 11.12). In
contrast, low-density solitary species such as the marine mammals
(whales and elephant seals), moose, or Tasmanian devils have
much less polymorphism. It is also of interest to note the case of the

cheetah, where some wild populations have reduced MHC class II
polymorphism as a result of recent population bottlenecks. Because
of this low MHC diversity, some cheetahs will accept allografts
from other, unrelated cheetahs. Likewise an infectious disease such
as feline infectious peritonitis causes 60% mortality in captive

cheetahs compared with 1% to 2% mortality in domestic cats.

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