Antisera used to identify blood groups (typing reagents) usually are produced as isoimmune sera. Their in vitro serologic characteristics vary with the species. Many reagents are hemagglutinins; others are hemolytic and require complement to complete the serologic reaction, such as in cattle (because RBCs do not readily agglutinate) and horses (because RBC rouleaux are a problem). Other typing reagents, neither hemagglutinating nor hemolytic, combine with RBC antigens in an “incomplete” reaction because they lack additional combining sites to agglutinate other RBCs; addition of species-specific antiglobulin is required for agglutination.
The diversity of blood groups in animals and the lack of commercially available blood-typing reagents to all antigens make complete typing difficult but should not preclude the clinical use of transfusions. In horses and dogs, the blood group antigens most commonly implicated in transfusion incompatibilities are known; by selecting donor animals that lack these groups, or that match the recipient, the risk of sensitization of the recipient to the most important antigens can be minimized. For dogs and cats, commercially available, point-of-care testing for major antigens is available in either gel or card testing kits. Reagents are available for only some antigens, generally those that are most likely to sensitize a recipient, or those for which naturally occurring antibodies, primarily in cats, might be present. For example, dogs have more than 12 blood group systems but generally are typed for only one (DEA 1.1). An additional blood group antigen (dal) was discovered when a dal-negative, previously transfused Dalmatian reacted to many potential donors, and only a few Dalmatians were found to be compatible. It is a common antigen in most dogs but is lacking in some Dalmatians. Because multiple blood group antigens are present, it is likely that an animal receiving a transfusion might be exposed to some antigens that are not present on its RBCs.