A new class of antibody-functionalized, semi-flexible and filamentous polymers (diameter 5-10 nm, length ∼200 nm) with a controlled persistence length, a high degree of stereoregularity and the potential for multiple simultaneous receptor interactions has been developed. We have decorated these highly controlled, semi-stiff polymers with T cell activating anti-CD3 antibodies and analyzed their application potential as simple synthetic mimics of dendritic cells (sDCs). Our sDCs do not only activate T cells at significantly lower concentrations than free antibodies or rigid sphere-like counterparts (PLGA particles) but also induce a more robust T cell response. Our novel design further yields sDCs that are biocompatible and non-toxic. The observed increased efficacy highlights the importance of architectural flexibility and multivalency for modulating T cell response and cellular function in general.