An animal’s blood group is determined by measuring the reaction of a small sample of blood to certain antibodies. Dogs are routinely typed only for the most potent antigen, DEA 1.1. In addition to DEA 1.1 at least 12 other blood group systems are present. Although the risk is less, any antigen might cause a reaction if those cells are given to a previously sensitized dog. Any dog that has had a previous transfusion may have antibodies to any of the blood group antigens not present on their own red blood cells. These antibodies can be detected by testing the red blood cells from a potential donor with plasma (the clear, yellowish liquid part of blood) taken from the recipient. This procedure is called a major crossmatch. If agglutination occurs, the recipient has antibodies that could destroy the donated red blood cells. That donor is incompatible and should not be used.