Vaccine titer testing is a way of measuring a pet’s immune system response when the pet is vaccinated against a specific disease. Titer tests detect antibodies, which are proteins produced by the body when the immune system detects a disease-causing organism (e.g., virus, bacteria) or another “foreign” substance, like a vaccine. Antibody-stimulating substances are called antigens. Titer test results tell your veterinarian not only whether your pet has antibodies to a specific antigen, but also the level of these specific antibodies.