Dogs have eight blood groups different from those of humans; and therefore with different nomenclature.
List of dog blood types
The system that classifies canine blood is the DEA (Dog Erythrocyte Antigen), which distinguishes eight groups: DEA 1.1, DEA 1.2, DEA 3, DEA 4, DEA 5, DEA 6, DEA 7, and DEA 8.
The difference between one group and another is in the membranes that surround the blood cells, the erythrocytes or red blood cells, which carry oxygen molecules through the body.
These membranes are also created by molecules called antigens, hence the name of the system: DEA. The antigens determine the blood group to which a dog belongs. Only a blood test is enough to know the group to which the animal belongs. The most common is the DEA 1.1
Canine blood transfusion protocol
The first time a dog receives incompatible blood he does not have the antibodies necessary to reject it, so nothing would happen. However, if he needs a second transfusion, you should only use blood from the same blood group. Although the danger is much higher in the case of the cats, the life of the dogs can also be at risk by not providing compatible blood for him.
There are several animal blood banks scattered across the United States. Many blood banks operate as a nonprofit business, recruiting volunteer donor dogs in the community. This website lists some of them with their contact information. If you are in the United Kingdom, here is a website that can locate a blood bank near you.
To be a donor, like people, dogs must be in good health and weigh at least 20kg.