In human medicine, anyone can receive blood donated by someone who is type O. Type O’s are known as universal donors; people with other blood types don’t develop antibodies to reject their blood. Not so with cats. They have three major types of blood: A, B, and AB. There’s no type O, so if your cat needs blood, the donation has to be matched with his type. “You can kill a cat by administering a quarter-teaspoon of type B blood to a type A cat,” says Elizabeth Rozanski, DVM, an emergency and critical care vet at Tufts.