[Biological and evolutionary significance of the major histocompatibility complex]

The recent advances in the understanding of the Major Histocompatibility Complex, particularly of human HLA and murine H2, are critically reviewed. Special emphasis has been given to the new hypotheses regarding MHC a "rejection" system for recognizing and eliminating cells that have been altered in the expression of their so called transplantation antigens by viral infection or oncogenic transformation. Because Ir genes are certainly involved in controlling the immune response against modified autologous MHC antigens, the author forwards an original hypothesis for explaining the linkage disequilibrium between different alleles at the HLA: A, B, C loci: the Ir gene (genes) in a given haplotype is particularly efficient for recognizing some particular altered antigens of different serie (say A1 and B8; A3 and B7). If these is the case the Ir gene (genes) would be the Keystone for maintaining the preferential associations of some pseudoalleles in some haplotypes.