From Phys.Org:
A perplexing question in immunology has been, how do immune cells remember an infection or a vaccination so that they can spring into action decades later? Research led by scientists at the University of California, Berkeley, in collaboration with investigators at Emory University, has found an answer: A small pool of the same immune cells that responded to the original invasion remain alive for years, developing unique features that keep them primed and waiting for the same microbe to re-invade the body.Before this study, scientists were not sure how cells can remember an infection from up to 30 years earlier. To tease apart this mystery, the research team tracked a specific kind of immune cell through the human body in the weeks, months and years following a vaccination that gives long-term protection. The researchers tracked T cells inside people's bodies after they were given the long-lasting yellow fever virus vaccine, using a technology developed at Berkeley for monitoring the birth and death of cells in humans over long periods of time. The researchers found that CD8+ T cells, responsible for long-term immunity against yellow fever, proliferate rapidly on exposure to the vaccine but then evolve, beginning about four weeks after the vaccination, into a "memory pool" of cells that live more than 10 times longer than the average T cell.
More here.