The concept of a “Major Histocompatibility Gene” was first introduced by Snell and collaborators in 1956 to make a distinction between gene(s) associated with acute rejection of allogeneic tissue and tumor grafts, from those which control chronic rejection of normal allogeneic tissue grafts and usually do not cause rejection of tumor grafts. The latter were called “Minor Histocompatibility Genes.” Today, we know that, in most species, there is not only one gene, but a cluster of genes, whose products are involved in acute (strong) transplantation reactions. This genetic region is therefore called the Major Histocompatibility Gene Complex (MHC) or System (MHS). It represents the most polymorphic and multiallelic genetic system known to date in mammals.
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