Upon maturation, dendritic cells (DCs) acquire the unique ability to activate naïve T cells. We used time-lapse video microscopy and two-photon imaging of intact lymph nodes to showthat after establishing initial contact between their dendrites and naïve T lymphocytes, mature DCs migrate toward the contacted lymphocytes. Subsequently, the DCs tightly entrap the T cells within a complex net of membrane extensions. The Rho family guanosine triphosphatases Rac1 and Rac2 but not Rho itself control the formation of dendrites in mature DCs, their polarized short-range migration toward T cells, and T cell priming.