Immunotherapy is a type of treatment designed to stimulate the immune system. The healthy immune system is designed to recognize the body's own normal cells (ie, red blood cells, white blood cells, and all other cells that an individual is born with) as "self" and foreign cells (ie, viruses, bacteria, and parasites) as "non-self" and dangerous. Because cancer cells originate from normal body cells, the immune system often does not recognize them as "non-self," which allows them to replicate out of control.

The ideal cancer immunotherapy agent stimulates the immune system to discriminate between cancer and normal cells and is potent enough to kill tumor cells. One example is the canine melanoma vaccine, which is the first commercially available vaccine for the treatment of canine melanoma. This innovative DNA-based cancer vaccine has significantly improved the lifespan of dogs that have received it. Since receiving full government approval in 2009, ONCEPT, produced by Merial, has been shown to be a safe and effective adjunct therapy for dogs with canine melanoma.

Additional immunotherapy available includes B cell lymphoma vaccine. The development of a B cell antibody vaccine sparked a revolution in the treatment of B-cell lymphoma in humans. Recently a vaccine to induce antibodies that target the same antigen (CD20) has been produced for canine patients. DNA cancer vaccines offer a safe adjunct to chemotherapy with the potential to induce longer-term remissions. This cancer vaccine is indicated for dogs diagnosed with large B-cell lymphoma upon achieving remission through chemotherapy. Initial studies of the vaccine have shown it to be safe and a small study suggested improved survival times (>734 days) for dogs receiving both chemotherapy and vaccine over dogs treated with chemotherapy alone (approximately one year survival time).