Immunological synapse formation is usually assumed to require antigen recognition by T cell receptors. However, the immunological synapse formed at the interface between naïve T cells and dendritic cells (DCs) has never been described. We show here that in the absence of antigen, and even of major histocompatibility complex molecules, T cell–DC synapses are formed and lead to several T cell responses: a local increase in tyrosine phosphorylation, small Ca²⁺ responses, weak proliferation and long-term survival. These responses are triggered more readily in CD4⁺ T cells than in CD8⁺ T cells, which express a specific isoform of the repulsive molecule CD43. These phenomena may play a major role in the maintenance of the naïve T cell pool in vivo.