The immune system can seek and destroy rogue cancer cells, but it must first identify its target. The body's antigen-presenting cells normally do this by collecting and displaying fragments of tumor proteins, showing other immune cells what to hunt down. The problem arises when tumors sometimes resemble healthy tissue, escaping the antigen-presenting drag-net. Green's nanoparticles play a similar role, but because they can be loaded with any type of protein, they can program the body to fight even the most elusive cancers. "We are training the immune system to recognize what cancer looks like," Green says.