Upon infection, adaptive immune responses play catch-up with rapidly replicating pathogens. While mechanisms for efficient humoral responses to lymph-borne antigens have been characterized, the current paradigm for T cell responses to infections and particulate vaccines involves delayed migration of peripheral antigen-bearing dendritic cells (DCs) to lymph nodes (LNs), where they elicit effector T cell responses. Utilizing whole LN 3D imaging, histo-cytometry, and intravital 2-photon microscopy, we have identified a specialized population of DCs, enriched in the LN-resident CD11b(+) subset, which resides within the lymphatic sinus endothelium and scans lymph with motile dendrites. These DCs capture draining particles and present associated antigens to T lymphocytes, inducing T cell responses much sooner than and independently of migratory DCs. Thus, strategic DC subset positioning in LNs limits a potentially costly delay in generation of T cell responses to lymph-borne antigens, contributing to effective host defense. These findings are also highly relevant to vaccine design.