Dogs have more than 13 blood types and can act as universal donors for their first transfusion.

The canine blood typing system is primarily categorized by the Dog Erythrocyte Antigen (DEA) system, which currently identifies over 13 distinct blood types. The most significant types include DEA 1.1, 1.2, 3, 4, 5, and 7. DEA 1.1 is the most critical because it is highly antigenic, meaning it is the most likely to cause a severe immune reaction if incompatible blood is introduced.Approximately 40% of dogs are positive for DEA 1.1, while those that are DEA 1.1 negative are classified as universal donors. This is because their red blood cells lack the specific proteins that trigger a recipient's immune system to attack. In emergency veterinary medicine, a dog can often receive a first transfusion from any donor because dogs do not possess naturally occurring alloantibodies against other blood types, unlike the human ABO system.The danger arises after the first transfusion through a process called sensitization. Once a dog is exposed to a foreign blood type, its immune system produces antibodies within 4 to 14 days. If the dog receives a second mismatched transfusion later in life, these antibodies will cause an immediate and potentially fatal hemolytic reaction. Veterinary clinics now use rapid typing cards and cross-matching tests to ensure safety, especially for dogs that have been pregnant or previously transfused.