In the last two decades, T cell function has been analyzed in vitro from many different angles, with a great deal of attention dedicated to the basic requirements of activation. During this time, a compendium of information has been collected and has proven to be invaluable. Paradoxically, very little is known about T cell activation and function in vivo. In the last decade, a number of models have been developed which allow the tracking of Ag-activated T cells in vivo and these studies have been instrumental in advancing the field of T cell biology. In particular, a new and emerging paradigm of T cell immunity is evolving.
