Doctors would like to use all sorts of nanoparticles in the body, for example to construct detailed images of anatomy and disease, and to deliver cancer-fighting drugs deep within tumor tissue. However, millions of years of evolution have equipped the body to identify and reject foreign particles, even nanoparticles. And so one major challenge in the use of nanomedicine has been the immune system’s unfortunate efficiency in responding to what it sees as infection, at best clearing nanoparticles before they can accomplish their goals, and at worst, leading to dangerous immune overreaction that creates side effects and serious risks.