Horses have seven primary blood group systems and over 30 different blood types.

Equine hematology identifies seven internationally recognized blood group systems: A, C, D, K, P, Q, and U. These systems are determined by specific antigens on the surface of red blood cells. Because these antigens can combine in various ways, researchers have identified more than 30 unique blood types across different breeds.The complexity is a major challenge in veterinary medicine, particularly during emergencies. While humans have naturally occurring antibodies against other blood types, horses generally do not. This lack of natural antibodies often allows a horse to survive a first 'blind' transfusion without a prior cross-match. However, the horse's immune system will then develop antibodies against the foreign blood cells.If a second transfusion is required later in life, the horse's immune system may recognize the donor blood as a threat. This can lead to neonatal isoerythrolysis in foals or life-threatening hemolytic transfusion reactions in adults. To prevent this, veterinarians use the 'major cross-match' technique to observe how the recipient's serum reacts to the donor's red blood cells.Standardized blood typing for horses was largely established through research by the International Society for Animal Genetics (ISAG). Today, certain breeds like Standardbreds and Thoroughbreds are more likely to have specific antigen combinations. Maintaining a database of 'universal' donors, typically horses that lack the Aa and Qa antigens, is a common practice in high-end equine clinics.