To initiate an adaptive immune response, rare antigen-specific naive CD4⁺ T cells must interact with equally rare dendritic cells (DCs) bearing cognate peptide-major histocompatibility complex (MHC) complexes. Lymph nodes (LNs) draining the site of antigen entry are populated by lymphoid-resident DCs as well as DCs that have immigrated from tissues, although the requirement for each population in initiating the T cell response remains unclear. Here, we show that antigen processing and presentation by both lymphoid-resident and migratory DCs was required for clonal selection and expansion of CD4⁺ T cells after subcutaneous immunization. Early antigen presentation by lymphoid-resident DCs initiated activation and trapping of antigen-specific T cells in the draining LN, without sufficing for clonal expansion. Migratory DCs, however, interacted with the CD4⁺ T cells retained in the LN to induce proliferation. Therefore, distinct DC subsets cooperate to alert and trap the appropriate cell and then license its expansion and differentiation.