After fighting off an infection, the body¿s immune system goes a step further: it remembers the invader so that the next time it detects that same pathogen, it can launch a swift and effective counterattack. That much is known. The details of how these actions are controlled, however, have remained largely mysterious. But a report published today in the journal Science is shedding light on the matter. According to the new study, two molecules previously known to take part in the fight against infections, perforin and interferon gamma, also direct the size and nature of the initial immune response and subsequent immune memory.

Microbiologist John Harty of the University of Iowa and his colleagues measured levels of immune system cells called T cells during the various stages of infection in mice that had been genetically engineered to lack either perforin or interferon gamma, or both molecules. Perforin, they found, controls the number of T cells created to fight the infection; interferon gamma takes care of eliminating most of those cells after they have conquered the invader. (About 10 percent of the T cells are maintained as memory cells.) Harty notes that this T cell death stage is particularly important because "it allows us to respond to many different pathogens without exhausting our immune system."

"Understanding how the basic biology of the system is regulated provides insight into how we might manipulate the system," Harty says. "In the case of these studies, the ultimate goal would be to learn how to manipulate the levels of T cell memory, which could result in better, more effective vaccinations."