Blood types are classifications of heritable species-specific antigens on the surface of red blood cells. Seven blood types are recognized in dogs, and four blood types are identified in cats. Other cells such as leukocytes, platelets, or cells in other tissues may also share these antigens. Alloantibodies (or isoantibodies) are antibodies present in the serum against an antigen from another animal of the same species. These may be naturally acquired (e.g. ingestion of colostrum) or induced through previous exposure (e.g. transfusion), and their presence is detected by a crossmatch.

Blood product transfusion may produce a wide range of harmful effects in veterinary patients. Some of these effects are common and may be unavoidable (e.g. fever), but others, such as immune-mediated acute and delayed transfusion reactions that are directly associated with inappropriate type and crossmatch processes in dogs and cats, can be minimized.

In this article, I present an overview of blood typing in dogs and cats and proper crossmatching techniques. I also offer decision-making recommendations for veterinarians to help avoid transfusion reactions, and I discuss the signs that may be observed if a reaction occurs.

Dog blood types are numbered according to the dog erythrocyte antigen (DEA) system.

DEA 1.1, 1.2, and 1.3

DEA 1 was formerly known as A and consists of four alleles: negative, 1.1, 1.2, and 1.3. DEA 1.1 is inherited as an autosomal dominant trait over DEA 1.2, and the null type is recessive to both. DEA 1.1 and DEA 1.2 are the most important antigens and together occur in about 60% of dogs.¹ Confusion may arise because both of these types have been considered A positive; however, DEA 1.2 dogs, which make up 7% to 29% of dogs, will develop potent anti-DEA 1.1 antibodies once transfused with DEA 1.1 cells.

While naturally occurring antibodies to these antigens are generally considered nonexistent, first-time transfusions with DEA 1.1 blood may be associated with a decreased circulating lifespan of transfused cells, and subsequent transfusions will be associated with an acute hemolytic reaction. Transfusion of DEA 1.2 blood to a sensitized DEA negative dog will result in an exponential loss of cells over the course of several weeks, with about half of the transfused cells being lost within the first 10 days.² DEA 1.3 is only known to exist in dogs from Australia, primarily German shepherds.³

DEA 4

DEA 4 occurs in up to 98% of dogs, and dogs with this type alone are considered universal donors. Only about 75% of Doberman pinschers are DEA 4 positive. Naturally occurring DEA 4 antibodies are not known to exist; however, hemolytic transfusion reactions can occur after sensitization with DEA 4 positive blood transfusions in dogs lacking that antigen.⁴

DEA 3 and 5

DEA 3 and 5 are expressed in lesser proportions of the dog population, but DEA 3 occurs in 23% of greyhounds, and 30% of greyhounds are DEA 5 positive. Naturally occurring antibody is present in 20% of DEA 3 negative dogs and 10% of DEA 5 negative dogs in the United States.²

DEA 7

DEA 7 is present in 8% to 45% of U.S. dogs. Naturally occurring antibodies have been observed against DEA 7, with a delayed transfusion reaction causing the decreased lifespan of transfused cells but no hemolysis.^5,6 While controversy regarding the importance of the antigen exists, it is best to avoid the premature loss of transfused cells by using donor blood lacking this antigen.