A new study reveals the geometric underpinnings of T-cell triggering through the precise engineering of T-cell receptor geometry in all three dimensions. The researchers used nanofabrication to create a biomimetic surface that simulates the key features of the APC, presenting T-cell receptor ligands in different geometric arrangements, with different inter-ligand spacings arranged in clusters of varying size. The results could have a significant impact on adoptive immunotherapy and the design of CAR T cells.