A couple of weeks ago Paul asked this and I'm finally getting around to it.
I wonder if you could discuss the issue of blood groups and blood transfusions in dogs.
I understand that large breed dogs often are blood donors and that unlike us humans, at least my understanding is, that a dog can receive a blood transfusion from another dog without the blood being of the same blood group as the donor's. Am I correct? How does this work? without killing the dog. Is it only once in a dog's lifetime that a transfusion of another blood group can be done? Are the blood groupings for dogs the same as humans?
Excellent question, Paul! I'll try to answer this question in a way that most laypeople can understand. All of my veterinary readers please be patient if I don't get into quite the detail that I know is out there. I'm also going to deal just with dog and cat blood types, as these are the most common pets owned and the ones I'm most familiar with.
For those who haven't quite caught on, yes, animals have blood types. Humans have four: A, B, AB, and O. When doing blood transfusions we have to "type" a person's blood and find out which kind it is. Type O is a "universal donor", meaning anyone can receive this type of blood, but those with Type O can only receive their on blood type. AB is the opposite, being a "universal acceptor" and can take any blood type but can only donate to other ABs. Humans naturally have antibodies against blood types other than their own, so if someone receives an incompatable blood type the body will not properly recognize it and will attack it as a "foreign" substance. This reaction can be serious and even fatal, which is why blood typing is important. This isn't really a concern with O blood because this type doesn't have the "triggers" on their surface that the other three have, so antibodies will not recognize or attack the cells.
Dogs have eight basic blood types, though as many as 12 may exist. The types are numbered and are structured differently than in humans. Dogs also are believed to not have naturally occurring antibodies against other blood types (again, different than in humans). This means that a first-time transfusion between two untyped dogs is very unlikely to cause a suddent transfusion reaction, even if their blood types are different. However, if the blood types are different, the recipient can then develop antibodies against the donor blood type. The next time this blood type is given, there is a chance of a reaction to the "foreign" blood.
Cats have three basic blood types, named A, B, and AB. AB is a rare blood type, though is found very commonly in the Abyssinian breed. Cats do have natural antibodies against different blood types, like in humans, but the severity differs. Type B cats have a high antibody level against Type A, and are likely to have a very serious reaction to even a single transfusion with A. Type A cats have lower levels against Type B and so reactions to B tend to be minor (however, these donor blood cells won't last as long in the body compared to A blood). AB blood can be safely given to cats of either other blood type.
So how is all of this relevant? Dogs can generally receive any other dog's blood for a transfusion if it's the first time the recipient has ever had a transfusion. After the first time it becomes important to type the blood to prevent a reaction. This is very nice in emergency situations as the vast majority of dogs have never received donor blood so if you have to do a quick transfusion you can really pick any canine blood sample. Cats aren't so lucky, as you need to type the blood of the donor and recipient every time or risk a potentially life-threatening reaction.
Many areas have pet blood donor programs, especially at veterinary colleges. In US there is also a national pet blood bank that can overnight blood to any clinic. Donors usually receive free health screening and testing to make sure they are good candidates for donating, since there is normally no compensation given for allowing your pet to be a donor.
Paul, this was a great question! Thanks for asking it!