"The immune system is as remarkable as it is complex. It is, after all, our body’s 24/7 watchdog. When the immune system functions normally, it distinguishes external foes from friends—mounting a powerful response to infections while allowing nourishing nutrients to enter our bloodstream. But as counter-intuitive as it might seem, the immune system sometimes turns against its host, sort of like a dog biting the hand that feeds it. When the immune system attacks the very body it’s meant to protect, it causes what doctors call autoimmune diseases."