Tiny, man-made spheres called nanoparticles can shuttle medicines to diseased tissues with incredible precision. But they all face a common challenge: The immune system sees the virus-size particles as threats, and eats them before they can reach their target. Previously, researchers had tried to dupe the immune system, with only limited success. So Liangfang Zhang borrowed a design from nature. He removed the membrane from a red blood cell and snipped it into pieces that he used to envelop nanoparticles. Because the membranes come pre-loaded with proteins that tell the immune system to back off, the cloaked particles slip past the body’s defenses.