T cells are essential to the immune system, but how do they distinguish between the body's own tissues and harmful invaders? Harvard MD/PhD candidate Dan Michelson's research reveals how the thymus, a small gland located in the chest, creates miniature versions of organs for T cells to "preview" so that they know friend from foe. Michelson’s findings deepen our understanding of autoimmune diseases like type 1 diabetes and rheumatoid arthritis with implications for therapies that strengthen the immune system and treat cancer.