LA JOLLA—Human bodies defend themselves using a diverse population of immune cells that circulate from one organ to another, responding to everything from cuts to colds to cancer. But plants don’t have this luxury. Because plant cells are immobile, each individual cell is forced to manage its own immunity in addition to its many other responsibilities, like turning sunlight into energy or using that energy to grow. How these multitasking cells accomplish it all—detecting threats, communicating those threats, and responding effectively—has remained unclear.