When puppies are born, they are protected by special antibodies produced in their mother's milk, but as they get older they lose this protection. (animals.howstuffworks.com)
Vets also look for evidence that white blood cells have produced special antibodies to fight distemper. (dog-health-guide.org)
Vets will also look for special antibodies that may have been produced to fight distemper. (dog-health-guide.org)