Why do MHC class I molecules evolve faster than class II ?
A general observation from comparing MHCs from different species is that the evolution of MHC class I molecules is much more rapid and exuberant than that of their class II counterparts. This is relatively easy to reconcile with the concept that MHC evolution is mostly driven by pressures imposed by infectious pathogens such as rapidly evolving viruses. Indeed, if a mutated virus acquires that capacity to evade or block MHC class I antigen presentation, the progeny of that virus will have an immediate advantage over the viruses infecting the adjacent cells. If the evasion mechanism concerns class II presentation, however, virions arising will still carry epitopes for TCR recognition by CTLs, and antigen presentation and local inflammation will still be taking place because the infection with unmutated viruses will still be happening in adjacent cells. The advantage of mutated pathogens having acquired the means to evade class II presentation will therefore only become effective if and
