Gene products of the human HLA region are the principal histocompatibility barrier to human allogeneic tissue transplantation1. In addition, HLA genes regulate immune responses to environmental antigens2 and affect susceptibility to various diseases3. Although the HLA region has been studied intensively, only four histocompatibility loci (HLA-A, -B, -C and -DR) have been identified within this 'supergene', but the existence of additional loci has been suggested by immunochemical4-9 and genetic recombination10-15 studies. Recently, a new segregant series of five histocompatibility antigens, named 'Secondary B cell' (SB) antigens, was defined16-18. We now report studies of 21 families which indicate that the gene encoding the SB antigens is closely linked to HLA-DR, but is distinguished from it by recombination in two families and can be mapped between HLA-DR and GLO in additional recombinant families.
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