Evidence of a second polymorphic ELA class I (ELA‐B) locus and gene order for three loci of the equine major histocompatibility complex

Summary. Two antisera, B‐442 and R‐2046, were produced by immunizing offspring with purified peripheral blood lymphocytes from a parent matched for the ELA‐A specificity carried on the unshared haplotype. Absorption analysis demonstrated that these antisera contained at least two families of cytotoxic antibodies, one directed against antigens present on T and B cells, and a second directed preferentially against antigens present on surface Ig positive cells. Immunoprecipitation studies using these antisera demonstrated that both antisera contain antibodies specific for glycoproteins with molecular weights characteristic of class I and class II MHC antigens. In lymphocyte typing tests of unfractionated lymphocytes, only the class I activity was readily detectable since the class II activity killed less than 25% of the cells. Family studies demonstrated that these antisera recognize products of genes linked to the ELA system. Based on two recombinants in an extended family it became apparent that the specificities detected by B‐442 and R‐2046 are not products of the ELA‐A locus, but rather they are products of at least one other locus, defined in this paper as ELA‐B. In this family a third recombinant was found between the A blood group system and the ELA‐A locus. Based on these three recombinants, the most probable linear relationship of the following genes is: A blood group system/ELA‐A/ELA‐B.