To sense their environment, cells rely on the receptor proteins that stud their surface. These receptors latch onto specific molecules, triggering a cascade of biochemical events that lead to cell behaviors, such as the secretion of hormones or the destruction of pathogens. But before receptors can switch on, they often have to bump into one another. Donald Ingber of Harvard Medical School and his colleagues demonstrated that they could control this activation using particles of iron oxide attached to dinitrophenyl (DNP) molecules, which attach to the receptors on histamine-producing mast cells. When magnetized, the 30-nanometer-wide beads would attract one another, forcing the receptors to huddle and activate. The researchers detected a spike in the calcium levels inside the cells, which is the first step in histamine secretion. The technique could lead to lighter-weight, lower-power biosensors for detecting pathogens or to novel ways of delivering drugs in the body. The work appears in the January Nature Nanotechnology.