Immunogenetics. 2016; 68: 401–416.
Published online 2016 May 27. doi: 10.1007/s00251-016-0918-x

HLA class I molecular variation and peptide-binding properties suggest a model of joint divergent asymmetric selection

… We thus propose that HLA-A and HLA-B evolved through a model of joint divergent asymmetric selection conferring all populations an equivalent immune potential…

It is on open access: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4911380/
Enjoy.