The immune system is like a roving brain that provides surveillance on the outside and inside of our bodies against invasion. When the immune system becomes under-reactive, we develop infections and cancer. When the immune system is over-active, we develop allergies and autoimmune disease. We can have too much (cardio-vascular disease), or too little (cancer). We are born with an innate immune system and over our lives the adaptive immune system learns about the environmental microbes that we encounter