The T cell receptor (TCR) mediates antigen recognition and T cell activation via its dimeric αβ, CD3εγ, CD3εδ and CD3ξξ subunits. A structural mechanism integrating both functions remains elusive. Here, anti-CD3 mAbs, NMR cross-saturation plus chemical shift mapping and mutational analyses were used to show that site-specific binding topology and TCR quaternary change are essential for activation. Two agonist mAbs footprint to the membrane distal CD3ε lobe, whereas a non-agonist mAb binds to the cleft between CD3ε and CD3γ. T cell triggering is not linked to mAb affinity or CD3ε binding stoichiometry per TCR but requires an intact TCRβ-CD3εγ module. CD3γ ectodomain mutants or an Fab directed toward the lever-like Cβ FG loop near the agonist mAb site inhibit antigen-dependent activation by preventing quaternary change upon ligand binding. Flexible elongated TCRα (25-26 aa) and TCRβ (19 aa) membrane stalks versus rigid compact CD3ε, CD3γ, and CD3δ (5-10 aa) G-strands and CxxC membrane proximal motifs permit dynamic TCRαβ movement over asymmetric CD3 heterodimers upon p-MHC binding. Extracellular mechanical torque on the TCR resulting from specific p-MHC binding during T cell scanning of APCs can initiate quaternary structure changes within the TCR, triggering downstream signaling. These findings suggest that the TCR is a specialized mechanosensor, converting mechanical energy from immune surveillance into a biochemical signal at no energetic cost.