Only 1 blood group system, the AB system, has been identified in cats. In this system, there are 3 blood types; A, B and AB. Similar to humans, the blood group antigens are defined by specific carbohydrates on erythrocyte membranes. A N-glycolyl-neuraminic acid determines the A antigen and a N-acetyl-neuraminic acid determines the B antigen with equal amounts of both acids being found on AB erythrocytes. Cats with blood group B lack a hydroxylase enzyme that converts N-acetyl-|
neuraminic acid to N-glycolyl-neuraminic acid. The blood group antigens are inherited as a simple autosomal trait with A being dominant over B. The inheritance of the AB allele is, as yet, unknown (it is not due to codominance of A and B). The percentage of cats that are A or B positive is breed-dependent (see table below). The overall incidence of A positive DSH and DLH cats varies between countries, with a higher incidence in the USA (94 to 99%) than in the United Kingdom (87%) and Australia (73%). AB cats are quite uncommon (5% in the United Kingdom and <1% in the USA and Australia).
Cats have naturally occurring antibodies (alloantibodies) which are responsible for potentially life-threatening transfusion reactions. In B cats, the anti-A antibodies are strong agglutinins and hemolysins, especially of the IgM class. In contrast, anti-B antibodies in type A cats are weaker agglutinins and hemolysins (and are of the IgG and IgM class). Type AB cats lack naturally occurring antibodies and can safely receive blood from either type A or B cats (universal recipients).
The half-life of transfused erythrocytes in matched feline transfusions (i.e. type A blood to a type A cat or type B blood to a type B cat) is 29 to 39 days. Transfusion of A blood into a B cat results in rapid destruction of the donated type A blood (mean half-life of 1.3 hours) with severe clinical signs (hypotension, defecation, vomiting, hemoglobinemia, neurologic depression) and even death. In contrast, transfusion of type B blood into A cats produces milder clinical signs and the transfused erythrocytes have a mean half-life of 2.1 days. Due to the presence of these naturally occurring antibodies, cats must be crossmatched before their first transfusion (especially in those breeds with a high incidence of type B or AB blood). In addition, neonatal isoerythrolysis can occur in kittens bearing the A or AB blood group antigen from a mating of B queens to an A or AB tom.