Blood groups are determined by the presence or absence of certain proteins or sugars found on the red blood cell membrane. The number of blood groups varies among domestic species. Normally, individuals do not have antibodies against antigens on their own red blood cells or against other blood group antigens of that species unless they have been exposed to them by a blood transfusion or pregnancy. In horses and in some other species, however, antibodies from one individual that react with antigens of another individual may exist without any prior exposure. In horses, antibodies will be produced in the mare against fetal red blood cell antigens if the fetus has a different blood type from the mare. This happens because the mare is exposed to the different (foreign) antigen when fetal blood passes through the placenta. This can be a problem in subsequent pregnancies if foals have this same foreign antigen inherited from the stallion because the mare will then have high levels of the antibody that react against the foal's blood type. The maternal antibodies can be transmitted to the foals in colostrum (first milk) and will destroy the foal’s red blood cells. The first foal with that blood type isn't harmed because the mare hasn't had time to create the antibodies before the foal nurses. This immune disorder in the foal is called neonatal isoerythrolysis Regenerative Anemias .
In horses there are 8 major blood groups: A, C, D, K, P, Q, U, and T. Of these, A, C, and Q seem to be the most likely to stimulate an antibody response when given to a horse that is negative for them. These groups are identified to aid in matching of blood donors and recipients and to identify breeding pairs potentially at risk of causing neonatal isoerythrolysis in their offspring. Because the expression of blood group antigens is genetically controlled and the ways in which they are inherited are understood, these systems also have been used to confirm pedigrees in horses. However in most cases, DNA testing has now replaced blood typing for paternity testing.
Blood typing is a procedure for testing an animal’s blood by measuring the reaction of a small sample of blood to certain antibodies. In horses, it is most practical to type potential donors in advance, as it is seldom possible to type recipients on an emergency basis. By selecting donors that lack the blood group antigens (A, C, and Q) most likely to be problematic, or that match the recipient, the risk of causing a transfusion reaction can be minimized. An additional test, called a crossmatch, which tests donor red blood cells with a recipient's plasma (the liquid part of the blood), is also available to identify possible reactions before breeding or 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 longterm anemia. Animals with blood clotting disorders often require repeated transfusions.
Blood transfusions must be given with care because they have the potential to cause adverse reactions in rare cases. The most serious risk of transfusion is immediate destruction of red blood cells. In horses, a complete match is rarely possible, and even when donor and recipient are compatible, red blood cells survive only 2 to 4 days. Other rare potential complications include fever and the spread of infections.