T cells are our immune system’s customised tools for fighting infectious diseases and tumour cells. On their surface, these special white blood cells carry a receptor that recognises antigens. With the help of cryo-electron microscopy, biochemists and structural biologists from Goethe University Frankfurt, in collaboration the University of Oxford and the Max Planck Institute of Biophysics, were able to visualise the whole T-cell receptor complex with bound antigen at atomic resolution for the first time. Thereby they helped to understand a fundamental process which may pave the way for novel therapeutic approaches targeting severe diseases.