Early evolution of MHC polymorphism.
There is unwarranted satisfaction with the view that MHC polymorphism evolved because there was a selective advantage in having a variety of MHC proteins to bind a variety of peptide subsets for presentation to T cells. While this may, in part, explain its maintainance, polymorphism may have evolved initially to reject foreign virus "grafts". The possession of similar membranes promotes aggregation between "like" cells, but it also promotes aggregation between the cells and viruses which retain membrane components of their previous host. The selection pressure afforded by hostile virus "grafts" would favour cells which developed polymorphic membrane components (since "like" will not aggregate with "not-like"). This polymorphism would have evolved before the appearance of multicellular organisms. Thus, the evolution of modern immune systems would have been imposed upon pre-existing polymorphic systems. A path this evolution may have taken involves the development of mechanisms for intracellular distinction between self and not-self.
Department of Biochemistry, Queen's University, Kingston, Ontario, Canada.
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Forsdyke
D005075:Biological Evolution; D007107:Immune System; D008285:Major Histocompatibility Complex / Q000235:genetics*; D011110:Polymorphism, Genetic / Q000235:genetics* / Q000276:immunology; D018406:Virus Physiological Phenomena