The thymus gland trains young blood cells to know which disease-carriers to attack and which cells to leave alone. Mature cells, called T cells, leave the thymus ready for battle. A team at the AIDS Research Center at Massachusetts General Hospital in Boston announced they had created a lab-grown thymus- a crucial first step on the road to rebuilding devastated immune systems. Poznansky's team is betting it will be able to manipulate artificial thymuses to produce disease-specific T cells. "It will be potentially useful for reconstituting the T cell system of a patient who has AIDS, because in AIDS you get both a depletion of T cells and damage to the thymus," says Poznansky.