Blood groups are determined by genetically controlled, polymorphic, antigenic components of the RBC membrane. The allelic products of a particular genetic locus are classified as a blood group system. Some of these systems are highly complex, with many alleles defined at a locus; others consist of a single defined antigen. Blood group systems, in general, are independent of each other, and their inheritance conforms to Mendelian dominance. For polymorphic blood group systems, an animal inherits one allele from each parent and thus expresses no more than two blood group antigens of a system. An exception is in cattle, in which multiple alleles are inherited.
Normally, an individual does not have antibodies against antigens present on its own or against other blood group antigens of that species’ systems unless they have been induced by transfusion, pregnancy, or immunization. In some species (people, sheep, cattle, pigs, horses, cats, and dogs), so-called “naturally occurring” isoantibodies, not induced by transfusion or pregnancy, may be present in variable but detectable titers. For example, Group B cats have naturally occurring anti-A antibody.
Also, circulating antibodies to animal blood group antigens may be induced by transfusion. With random blood transfusions in dogs, there is a 30%–40% chance of sensitization of the recipient, primarily to blood group antigen DEA 1, but risk is also present for development of antibody to any other antigen lacked by the recipient. In horses, transplacental immunization of the mare by an incompatible fetal antigen inherited from the sire may occur. Immunization also may result when some homologous blood products are used as vaccines (eg, anaplasmosis in cattle). In dogs, prior pregnancy does not result in sensitization of the bitch to foreign blood group antigens.
The number of major recognized blood group systems (see Table: Major Blood Groups of Clinical Interest) varies among domestic species, with cattle being the most complex and cats the simplest. Animal blood groups are typed to aid in the matching of donors and recipients and, especially in horses, to identify breeding pairs potentially at risk of causing hemolytic disease in their offspring. Because expression of blood group antigens is genetically controlled and the modes of inheritance are understood, these systems also have been used to substantiate pedigrees in cattle and horses; however, in most cases, DNA testing has replaced blood typing for paternity testing.