Blood groups are determined by the presence or absence of certain antigens (proteins or sugars) found on the red blood cell membrane. The number of blood group systems varies among domestic species. Cats have 4 known blood types, A, B, AB, and mic. Type A is most common (about 99% of cats in the United States are Type A). Certain breeds have a higher prevalence of Type B. Type AB is rare. There are no universal donors among cats, because they naturally have antibodies against the blood group antigen that they lack. Patients are typed to aid in the matching of donors and recipients and to identify breeding pairs potentially at risk of causing hemolytic anemia in their offspring. Type B queens have especially strong anti-A antibodies. If they have kittens that inherit Type A from the tom, the antibody goes to the kittens in the colostrum (first milk) and destroys red blood cells in the kitten (see Anemia in Cats : Neonatal Isoerythrolysis).
An animal’s blood group is determined by measuring the reaction of a small sample of blood to certain antibodies. The blood group must be determined before a blood transfusion can be safely provided. In addition, a test called crossmatching is also necessary to ensure safe transfusions. In this test, specific components of donor blood are combined with components of the patient's blood prior to a transfusion to ensure that a reaction does not occur. Even the first incompatible transfusion results in the rapid destruction of the transfused cells.
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 of other causes. Animals with blood clotting disorders often require repeated transfusions. Transfusions of whole blood, red blood cells, plasma, platelets, or blood substitutes may be given. All transfusions must be given with care because of the potential for adverse effects. The most serious risk is destruction of the red blood cells shortly after they are given, called hemolysis. Fortunately, this rarely occurs in domestic animals. Other complications include transmission of infected blood or diseases (such as feline leukemia or immunodeficiency viruses) from an infected donor. Donors should be tested before blood is taken. If too much blood is given, edema (fluid) in the lungs could occur. Other less common or minor reactions include decreased calcium, fever, hives, or vomiting.
Also see professional content regarding blood groups and blood transfusions.