"Memory" T cells are the immune system's primary defenders against antigens. When the immune system first comes
in contact with an antigen-such as the viruses that cause chicken pox, measles or polio-it creates killer T cells
that then turn into "memory" T cells. If and when the antigen invades the immune system a second time, these
"memory" T cells recognize the invader and bind to it, killing the antigen more quickly than during the first
exposure. "You get a nuclear holocaust, not just gunfire," said Dr. Marrack, a National Jewish researcher who
studies the inner workings of the immune system. (This is why people don't get chicken pox, measles or similar
immune diseases more than once.)

Still, researchers were unsure if T cells maintained the ability to remember antigens from one exposure to another-there
could be decades between exposures-by laying in wait, by being exposed to antigens regularly or by holding on to small
parts of the virus. Recently, National Jewish researchers found that none of these explanations were quite right. Rather,
"memory" T cells stay alive for many years because they divide slowly, stimulated by a special hormone (cytokine) of the
immune system, IL-15.  Another immune system cytokine, IL-2, prevents this IL-15-induced production from getting
out of control.

Working together, IL-15 and IL-2 help to provide equilibrium to the immune system. When an immune response to an
antigen is mounted, IL-15 is produced, which causes T cells to divide and attack the invader and stimulates "memory"
T cells. At the same time that IL-15 production increases, IL-2 controls the proliferation of "memory" T cells-caused by
increasing amounts of IL-15-by killing some T cells as they divide.