David Masopust has long imagined how to push immune systems to their limits—how to rally the most powerful army of protective cells. But one of the big mysteries of immunology is that so far, nobody knows what those limits are. So he hatched a project: to keep mouse immune cells battle-ready as long as possible. “The idea was, let’s keep doing this until the wheels fall off the bus,” says Masopust, a professor of immunology at the University of Minnesota.