The prevailing view of signal transduction is that it proceeds through the linear relay of information via sequential bimolecular interactions, involving, for example, Src homology (SH) 2 domains. It has been assumed that such interactions are highly selective, i.e. that the affinities of these interactions are several orders of magnitude higher than that for non-specific interactions. However, recent studies have suggested that the difference in affinities between so-called specific and non-specific interactions is not sufficient to support such a proposal. This therefore raises the question of how signalling pathway specificity is generated at all. To address this, we have taken a systems approach by expressing and purifying >90% of the SH2 domains identified in a T cell line using a next-generation sequencing-based transcriptomic analysis, and performed a systematic survey of the interaction of these SH2 domains with a set of potential phosphorylated peptides derived from the key signalling receptors of the T cell (including CD28, CTLA-4, PD-1, ICOS, BTLA, LAT and the CD3 subunits of the TCR complex), using surface plasmon resonance-based binding assays. Our results show that, instead of being highly selective for certain SH2 domains, the T cell-expressed receptors are very cross-reactive, such that each receptor is found to interact with ~50 different SH2 domains on average. In silico analysis based on these results confirms the expectation that affinity itself is not the sole determining factor for receptor specificity. Further exploration of the system using in silico simulations incorporating the absolute concentrations of SH2 domain-containing proteins measured in T cells using a proteomics-based approach, suggests instead that the specificity of SH2 domain recruitment by T-cell receptors is the result of systems effects, with expression levels of the signalling proteins being a major factor. Surprisingly, LCK, the most highly expressed SH2 domain in resting Jurkat, is predicted to dominate the binding of most receptors, suggesting a novel mechanism of Src kinase activation and function.