Your thymus is a walnut-sized gland that sits just above your heart. The master gland of the immune system, one of the thymus’ chief functions is to produce T lymphocytes, which are a type of white blood cell that works to prevent disease. In July 2000, a team of Harvard Medical School researchers at Massachusetts General Hospital, including Associate Professor of Medicine David Scadden, unveiled a man-made structure that mimics the thymus by churning out human T cells. The building blocks for this “organoid” were CellFoam, a synthetic material manufactured by Cytomatrix of Woburn, Mass., and tissue from humans and mice. The researchers are at work on an artificial thymus that would use exclusively human cells, which could come from donors or from the cells of, for instance, someone who needs an organ transplant. Such systems could open a dizzying array of possibilities, not only in the area of organ transplants but also in AIDS and cancer.