Cats have one major blood group system: the AB blood group. Blood groups are determined by the presence of antigenic markers on the surface of RBCs; cats are either blood type A, B, or AB. Transfusion reactions stem from from naturally occurring alloantibodies in the recipient’s blood against a blood-type antigen in donor blood. Correct identification of feline blood types is necessary to prevent potentially fatal transfusion reactions. Type B cats receiving type A or AB blood can have a potentially fatal hemolytic transfusion reaction as type B cats have high anti-A alloantibody titers. Type A cats have low titers of anti-B alloantibodies, but can still experience premature destruction of RBCs if they receive type B or AB blood.