The most important internal defense that your dog has to fight off disease causing bacteria, toxins, parasites, and viruses is the immune system. The dog immune system has two major components: the skin and the adaptive immune system. When the dog immune system malfunctions, it can attack its own body (called autoimmunity), overreact (called hypersensitivity), or react insufficiently (called immunodeficiency or suppression). Vaccinations help optimize your dog's immunity, as well as diet and exercise. Learn more about your dog's immune system and the illnesses that your dog is more susceptible to without his proper defenses in place.