The laboratory recombinant haplotype H-1acl of the Norway rat has been used in studies of the rejection and passive enhancement of kidney allografts. While the full H-1a haplotype provoked acute rejection, neither of the isolated subregions, H-1Aa and H-1Ba, did so. It was also found that alloantisera raised against either the H-1Aa or the H-1Ba antigens would enhance the grafts. It is suggested that both MHC subregions contain a histocompatibility locus (i) for kidney (as they do for skin) and that in the genetic combinations studied only incompatibility for both provokes a response sufficient for rejection. In other combinations, however, single region incompatibilities may be sufficient.

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