Why do humans have three different blood groups?

Karen Humm, Emergency and Critical Care Vet, Queen Mother Hospital for Animals, London.

All animals do have different blood groups. Humans have a system based on A, B and O. Other animals have different systems. Basically the blood groups are based on little proteins that sit on the outside of red blood cells. The reason it's important for us to be able to tell what blood groups an animal or a person is in is because the blood from one person may not be compatible with another person if their blood group is not the same. In dogs we have a system called DEA 1.1, 1.2, 3, 4, 5, 6 and 7. In cats we also have A and B but they're not the same as A and B in human blood systems. We don't know exactly why people have different blood groups or why animals have different blood groups but it is very important because of the risk of interactions with different blood groups in people and other animals. It might be that some blood groups give an advantage in some circumstances and certainly some ethnic groups are more likely to have blood groups than other people. People from Mediterranean origin are much more likely to have blood group B than people from non-Mediterranean background. The actual causes of them we don't fully understand.

*And Karen also made an appeal for doggy blood donors. There is currently a UK-wide shortage of blood for canine transfusions but you can take your pet down to the vet to help out - they won't take a whole pint!

Dr John Gibson, Veterinary School, University of Cambridge

The reason for ABO groups in humans is not really known but they are of vital consideration for transfusions. They are proteins on the surface of red cells. They are inherited from parents and differ between individuals

- so you can be A or B or AB or have neither (= O). In the blood there are naturally occurring antibodies which react with the blood group proteins which you don;t have. So if you are group A, you will have antibodies against group B. The antibodies will react if they see red cells with the group that they target. So a group A person needs to avoid transfusions from a group B person. As well as ABO groups, there are lots of other blood groups like Rhesus and Duffy. To most of these, there are not naturally occurring antibodies, so they are less important for a single transfusion - but the antibodies can be made following a transfusion.

But the reason for these groups is not really known. There is some association with diseases but it is weak for ABO and no real function for the proteins is known. Some other groups are proteins which do do things like act as membrane transport systems or dockage points for parasites.

Animals do not have the human ABO group on their red cells. But they do have other blood group proteins which are sometimes similarly important for transfusion. Dogs have DEAs (dog erythrocyte antigens), about 8 important ones of which DEA4 and 6 are most significant for transfusions. Cats have groups A, B or AB (but they are not quite like human ABO). Horses have groups including A, C, D, K, P, Q and U. Sheep A, B, C, D, M, R and X. Goats A, B, C, M and J. And so on. Whether these are important for transfusions depends on whether naturally occurring antibodies are present. As for human, its worth testing whether an animal's plasma has antibodies against a potential donor animal's blood.