The blood type of a dog is different from humans.
The determination of blood type is based on the proteins, or antigens, that cover the surface of red blood cells. Antibodies within an organism do not react the same to all blood proteins, meaning they do not react the same to all blood types. This is why it is important to know the blood type of an animal before performing a blood transfusion as the donor's blood might not be accepted by the recipient's.
Blood typing for humans is designated mainly according to the ABO naming system in which there are four kinds: A, B, AB, and O, each having positives and negatives. An A-type will create antibodies against a B-type, in which case a potentially fatal reaction would occur if an A-blood person receives B-blood. This reaction is called agglutination, which basically means clumping of the blood cells.
In dogs, the system works the same but the blood types are different. For canines, the most important is the DEA group, which stands for dog erythrocyte antigen. (Red blood cells are scientifically called erythrocytes, and antigen refers to the blood protein molecule that is capable of inducing an antibody immune response.)
Though there are over 13 groups--eight within DEA--dogs are most often typed just for DEA 1.1 positive or negative. DEA 1.1+ can be considered a universal recipient, much like human AB+ blood type, whereas DEA 1.1- is a universal donor, similar to O- blood type in humans. Other DEA blood types include 1.2, 1.3, 2, 3, 4, 5, and 7, all of which can be positive or negative.
Unlike humans, dogs do not have naturally-occurring antibodies of the opposing blood types in their bodies before a transfusion. For this reason, the first blood donation for a dog can typically come from any blood type, but thereafter, the donor blood will have to match to prevent possible complications.
To determine blood typing in canines is simple. Essentially, the process screens for specific antibodies whose presence or absence acts as recognition for that blood type. Drops of blood and saline in sample boxes on a specialized blood type sheet will either agglutinate or not agglutinate. If they agglutinate in the patient test box (which has a reagent with DEA 1.1+ antibodies), then the dog has blood type DEA 1.1+.
Crossmatching takes blood compatibility a step further to see how blood of the recipient and potential donor will react. If the donor and recipient blood clump together in a test tube, then the recipient's blood has antibodies that would attack the donor red blood cells. In such a situation, another blood donor is required.