This letter investigates the potential of pinching-antenna systems for enhancing physical layer security. By pre-installing multiple pinching antennas at discrete positions along a waveguide, the capability of the considered system to perform amplitude and phase adjustment is validated through the formulation of a secrecy rate maximization problem. Specifically, amplitude control is applied to enhance the signal quality at the legitimate user, while phase alignment is designed to degrade the received signal quality at the eavesdropper. This cooperation among pinching antennas is modeled as a coalitional game, and a corresponding antenna activation algorithm is proposed. The individual impact of each antenna is quantified based on the Shapley value and marginal contribution, providing a fair and efficient method for performance evaluation. Simulation results show that the considered pinching-antenna system achieves significant improvements in secrecy rate, and that the Shapley value based algorithm outperforms conventional coalition value based solutions.