The immune system is made up of specialized organs, tissues and cells whose job it is to seek out and destroy foreign invaders. The most common foreign invaders are microbes such as bacteria, parasites and fungi that can cause infection. The immune system is incredibly complex and there are two particular ways in which it can fail: an overactive or underactive immune response. Sometimes the immune system can attack unharmful invaders in your dog’s body, such as certain foods, the result is an allergic reaction. On the contrary, sometimes the immune system may miss the mark on a harmful invader, allowing it to manifest, such as in the case of cancer. Keeping your dog’s immune system strong can be helpful in preventing or treating these issues among many others.