Blood types are proteins, carbohydrates or fatty substances located on the surface of the red blood cells circulating throughout our bodies. These compounds are important as they can induce the immune system to react to foreign “invaders” such as bacteria or viruses or anything that the body determines as not normal. These foreign substances are known as Antigens (the bad guys). Our defense mechanism produces white blood cells (lymphocytes) who in turn make Antibodies (our good guys). War ensures, and the antibodies attack and attach to the antigens. This reaction causes the body to develop large white blood cells known as macrophages to destroy the foreign intruders. This response helps prevent and destroy infection, but it also leads to the destruction of red blood cells that have antigens. The same thing is true if the wrong blood type is introduced into the body via a blood transfusion . Dogs and cats do have different blood types, but not as many as people. Humans have over 30 different blood types while dogs and cats have 13 and 6 respectively. It is important to know the blood type if a transfusion is being considered in order to reduce the risk of a transfusion reaction. Some reactions are life threatening, especially in cats, where one transfusion of the wrong type can be fatal. Although there are 6 identified feline types, over 90% of cats in America are type A, only 9 % are type B and the rest are type AB. In dogs, although over 13 canine blood groups have been identified, 98% of the dog population in America have one of the most common types, namely, DEA 1.1. Basically a dogs that is DEA 1.1 positive is considered to be a universal recipient while DEA 1.1 negative dogs are considered to be universal donors. In dogs, it is not the first transfusion where complications can arise, but the second, if the wrong type is used. Veterinarians can do a simple blood test to determine if two blood samples are compatible prior to a transfusion. Blood type compatibility is critical for a transfusion to be successful.