Peering through a microscope at a plate of cells one day, Ralph M. Steinman spied something no one had ever seen before. It was the early 1970s, and he was a researcher at the Rockefeller University on Manhattan’s Upper East Side. At the time, scientists were still piecing together the basic building blocks of the immune system. They had figured out that there are B cells, white blood cells that help to identify foreign invaders, and T cells, another type of white blood cell that attacks those invaders. What puzzled them, however, was what triggered those T cells and B cells to go to work in the first place. Steinman glimpsed what he thought might be the missing piece: strange, spindly-armed cells unlike any he had ever noticed.