Blood groups are determined by the presence or absence of certain antigens (proteins and sugars) on the red blood cell membrane. Normally dogs do not have antibodies against any of the antigens present on their own red blood cells or against other canine blood group antigens unless they have been previously exposed to them by transfusion. In some species (such as humans and cats), however, antibodies from one individual that react with antigens of another individual of the same species may be present without any prior exposure.

Dogs have many blood groups, and their red blood cells may contain any combination of these since each blood group is inherited independently. The most important of these is called Dog Erythrocyte Antigen (DEA) 1.1. Typing of blood donors and recipients is done before transfusion. Approximately 40% of dogs are positive for DEA 1.1, meaning that they have that antigen on their red blood cells. By selecting donor animals that lack DEA 1.1 or that match the recipient, the risk of sensitizing the recipient can be minimized. If a dog is DEA 1.1-negative and is given DEA 1.1-positive blood, it may develop antibodies that rapidly destroy the red blood cells if a second DEA 1.1-positive transfusion is given. A DEA 1.1-positive dog may receive either positive or negative blood.

Blood Typing

An animal's blood group is determined by measuring the reaction of a small sample of blood to certain antibodies. Dogs are routinely typed only for the most potent antigen, DEA 1.1. In addition to DEA 1.1 at least 12 other blood group systems are present. Although the risk is less, any antigen might cause a reaction if those cells are given to a previously sensitized dog. Any dog that has had a previous transfusion may have antibodies to any of the blood group antigens not present on their own red blood cells. These antibodies can be detected by testing the red blood cells from a potential donor with plasma (the clear, yellowish liquid part of blood) taken from the recipient. This procedure is called a major crossmatch. If agglutination occurs, the recipient has antibodies that could destroy the donated red blood cells. That donor is incompatible and should not be used.

Blood Transfusions

Often, the need for a blood transfusion is an emergency, such as severe bleeding or sudden destruction of red blood cells due to other disease. Transfusions may also be needed to treat anemia. Animals with blood clotting disorders often require repeated transfusions of whole blood, red blood cells, plasma, or platelets. The most serious risk of transfusion is acute destruction of red blood cells, usually caused by a previously formed antibody to DEA 1.1, or to another antigen. Fortunately, this is rare. A more common problem in dogs that have received multiple transfusions is delayed destruction of the red blood cells, caused by antibodies to some of the minor blood group antigens.

Other complications of transfusions include infection from contaminated blood, a decrease in blood calcium levels, and accumulation of fluid in the lungs as a result of giving too large a volume of blood. Skin hives, fever, or vomiting are seen occasionally. Fortunately most transfusions are safe and effective.

Last full review/revision July 2011 by Peter H. Holmes, BVMS, PhD, Dr HC, FRCVS, FRSE, OBE; Nemi C. Jain, MVSc, PhD; David J. Waltisbuhl, BASc, MSc; Michael Bernstein, DVM, DACVIM; Karen L. Campbell, MS, DVM, DACVIM, DACVD; Timothy M. Fan, DVM, PhD, DACVIM; Wayne K. Jorgensen, BSc, PhD; Sarah E. Payne, DVM, DACVIM