PHILADELPHIA – An intruder is inside your body.

Maybe it’s a parasite from dirty drinking water. A virus from a coworker’s sneeze. Or a bacterium that sneaked in when you cut your finger.

Luckily for you, the immune system determines just which one of its many weapons will best repel the intruder, and what’s more, it “remembers” how to do the job even better, and faster, next time.

This phenomenon of immune memory has been recognized since at least the time of the ancient Greeks, yet no one could figure out how it worked.

Until Steven L. Reiner came along.

The University of Pennsylvania physician electrified his field last year by showing how the immune system generates two types of the sophisticated tools known as T-cells: one to fight invaders to the death, the other to remember the battle plan for the next time the same enemy shows up.

Reiner’s finding, made with John Chang, Vikram Palanivel and colleagues, was named one of the top 10 breakthroughs of 2007 by the journal Science. Using mice, the team provided evidence that T-cells arise from a self-renewing process like that used by stem cells. Without it, we’d fight off a bug once, and the next time we’d be dead.

“It’s an amazing system,” Reiner marvels. “You do use these cells, but you don’t deplete them. That’s how we can live long lives with short-lived cells.”

If all his team did was to illuminate the essence of a vital bodily defense mechanism – potentially leading to better vaccines – it would be big. But the research also may offer clues to two other medical enigmas: how the array of different blood cells is created in the bone marrow, and how cancer stem cells generate the cells in a tumor.

Among fans of the research is Amy J. Wagers, a specialist in blood stem cells at the Joslin Diabetes Center and the Harvard Stem Cell Institute.

“It’s beautiful science on its own, and it also has these far-reaching consequences in the field that I’m involved in,” she says.

Growing up in Utica, N.Y., Reiner was good at all academic subjects, but he was especially fond of digging into a contentious topic. An outspoken teen, he was captain of his high school debate team.

As an undergraduate at Haverford College, Reiner majored in philosophy but also was interested in psychiatry.

Several close friends were going to med school. Reiner’s mother and his grandfather were physicians. And his older brother Alex was studying to be a doctor. Ultimately, Reiner chose the same path.