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.
His intuition turned out to be correct. These dendritic cells, as Steinman named them, are now thought to play a crucial role in detecting invaders in the body and initiating an immune response against them. They snag interlopers with their arms, ingest them and carry them back to other types of immune cells—in effect, “teaching” them what to attack. It was a landmark discovery that explained in unprecedented detail how vaccines worked, and it propelled Steinman into the top tiers of his profession.