Polymorphism of class I antigens has been studied using the classical immunogenetic approaches: transplantation, serology, allospecific T-cells, and one-dimensional isoelectric focusing (1D-IEF). The serological approach for elucidating BoLA polymorphism was highly successful, resulting in the initial characterisation of the system. Sera produced by skin transplants, injection of leukocytes, and stimulation by pregnancy revealed the presence of a highly polymorphic MHC for the species. International workshops have played a vital role in standardisation of serological reagents, typing methods, and nomenclature of the BoLA system. The five international BoLA workshops during the period 1978 to 1992 elevated the status of the BoLA system to unparalleled leadership among the domestic animal species. More than 50 BoLA antigens have been recognised by the international community, nearly all of which behave as alleles of a single locus. The Fifth International BoLA Workshop (BoLA5) held at Interlaken, Switzerland in 1992, produced acceptance of 27 full-status BoLA specificities (e.g., BoLA-A2), 25 provisional workshop (w) BoLA specificities, and 1 specificity that may be encoded by a closely linked locus. (Table 3). All full-status BoLA-A antigens are defined by sera from at least two laboratories and have been recognised on two or more cells submitted to one of the international cell exchanges. Eleven specificities are "supertypic", i.e., they are shared by more than one allelic product. The BoLA specificities exhibit distinct differences in frequency in the different cattle breeds.

A lingering question is whether the sera commonly employed for BoLA typing are specific for the allelic products of a single locus or contain antibodies to the products of other call I loci as well. It has been suggested that typing sera are haplotype specific due to a high frequency of gametic phase disequilibrium between class I alleles at the population level. Within a cluster of sera that defines a BoLA-specificity an individual serum may have extra reactions that correspond to a second polymorphic class I locus. However, because this has never been shown for an entire cluster of BoLA typing sera, it is generally assumed that the primary specificities defined by these sera are alleles of a single locus. The large diversity of breeds represented in the international workshops, which one would expect to contain the class I alleles in different haplotypic combinations, have not been able to dissect the vast majority of sera used in the comparison tests. Therefore, although some sera that define a BoLA-A specificity may themselves be polyspecific, the BoLA-A specificities behave as alleles of a single locus.

This text was taken from Lewin 1996, with permission from the publisher CRC Press.