A blood transfusion can save a pet’s life. But just as in people, a lot depends on the blood type.

Blood types have to do with the proteins, or antigens, on the outside of red blood cells, also known as erythrocytes. Dog blood is typed using the Dog Erythrocyte Antigen — or DEA — system. About 50 percent of dogs have DEA 1.1 and so are DEA 1.1 positive, while the other half don’t have the antigen and are DEA 1.1 negative.

It’s another story in cats. They’re either type A, type B, or type AB. Although the same letters are used, these are different antigen types than in people.

If a negative dog receives blood from a positive dog, the immune system sees the cells as foreign and can attack them. When that happens, a blood transfusion can be fatal. That’s why matching donor and recipient blood types makes all the difference.