comparison of mammali.n proteins 1>.,. demonstrated thai divergence is highest amOllg the ligands and """plors invoh'ro in kosi defence mechanisms'. These include cytokines, <y_ tokin" n:-ceptors and leukocyte surface anli_ gens. There/Qre. it does not <:orne as a SUTprise that only a rel3li'ely low proportion, estimated to be .bout 7~. of mouse monodonal antibodies (mAbs) to human differ-enUation antigen. """'Snize homologous ",minanl molecules, Collaborative studies on human leukocyte antigens have cMbled tremendous ad,'anccs to be made in the understanding of the function of these cell-su.face molecules, allowing modulation of the immunesyslcm,and analysis.nd trealm",,1 of disease. Immunologists investigating """"ies other than humans have been rompelled to establish new sets 01 .nAbs and to assign a practical nomenclature. Three interna tional workshops have been held unde' the auspic~'S 01 the Vet~rinary Immunology Committee (VIC) 01 the [nternational Union of Immunological Sodeti~'S (lU IS) with the goals of comparing mAbs to leukocyte antigens 01 caUle, sheep and other ruminants, characterizing the .ntigens and assigning a systematic nomenclature (Hannover,