The immune system is primarily directed against invading microorganisms; however, it will also destroy abnormal host cells. For example, cytotoxic T cells Cell-mediated Immunity Innate immune responses, although critical to the defense of the body, cannot guarantee protection. They lack the flexibility to respond optimally to a diverse set of microorganisms, and they... read more kill virus-infected cells. Organ graft rejection is another example of a situation where cells that differ from normal are killed by T cells.
Under some circumstances the immune system can also kill cancer cells. In fact, for cancer to develop, it must first succeed in avoiding immunologic attack. One method of treating cancers is therefore to activate the immune system so that cancer cells are destroyed by cytotoxic T cells.
Cancer immunotherapy can take many forms. For example, cancer cell antigens may simply be used as vaccines. A licensed xenogeneic DNA-based vaccine is directed against the tyrosinase found in canine melanoma cells.
Passive immunization will also work. For example, monoclonal antibodies directed against CD20, a cell surface antigen found on canine lymphoma cells, have produced positive clinical responses.
Many tumors are highly immunosuppressive. They turn on regulatory pathways that block cytotoxic T cell activities.
This immunosuppression can be overcome by the use of monoclonal antibodies that block these immunosuppressive pathways—so-called checkpoint inhibitors. Checkpoint inhibitors have revolutionized the treatment of some human cancers, with promise for applications in veterinary oncology.