This article might be useful for dogs and cats seeking blood donation: http://www.theveterinaryexpert.com/blood-transfusions/dog-and-cat-blood-types/
What happens if a dog receives blood of a different type to their own?
The reason blood types are considered important is that there is a split in the canine population with about half being DEA1.1 positive and half being DEA 1.1 negative. The immune system of DEA 1.1 negative dogs recognises DEA 1.1 as a foreign substance and so attacks it. This can lead to severe transfusion reactions if a DEA 1.1 negative dog is given blood from a DEA 1.1 positive donor.
Just like in most illnesses, the immune system takes longer to react the first time it encounters something and so there is unlikely to be an immediate problem if a dog receives a mismatched blood transfusion the first time.
If a DEA 1.1 mismatched transfusion happens for a second time, the immune system is already prepared for the foreign substance and so the reaction can be quite severe, even life threatening.
What happens if a cat receives blood of a different type to their own?
Similar to dogs, mismatched blood transfusions can lead to life threatening reactions but the situation is a little different as the immune system in cats is pre-prepared and can react badly the first time there is a mismatch.