The immune system plays a central role for human health. Critically involved for its functioning is the activation of T-cells that relies on a complex intracellular signalling-network. To understand its behaviour, one has to study it in detail. Only then can one understand its dysfunction, as e.g. in case of tumors or infections, and develop novel drugs. Scientists at the MPI for Immune Biology have used biochemical and systemsbiology approaches to understand the function of the T-cell signalling-network. Of particular interest is the first element of the network, the T-cell antigen receptor (TCR-CD3) complex that recognizes foreign proteins.