Activation of antigen-specific T cells by mature dendritic cells in secondary lymphoid organs is a key control point of the adaptive immune response. Here we describe the ex vivo isolation of preformed multicellular clusters between T cells and antigen-presenting cells. Adoptively transferred, antigen-specific T cells segregated into individual clusters where their activation and proliferation was initiated in vivo. Transit of the T cell cohort through the cluster compartment required 32–36 h. The precise timing of the response to agonistic epitopes was remarkably invariant regardless of the T cell lineage, the major histocompatibility complex haplotype, and the antigen dose. Interestingly, initiation of cell division of T cells specific for a subdominant epitope and a weak agonist was delayed by 6 h. The results provide a basis for the analysis of short range, mutual cell–cell interactions within such confined microenvironments.