Why must T cells be cross-reactive?

Clonal selection theory proposed that individual T cells are specific for a single peptide-MHC antigen. However, the repertoire of alpha beta T cell receptors (TCRs) is dwarfed by the vast array of potential foreign peptide-MHC complexes, and a comprehensive system requires each T cell to recognize numerous peptides and thus be cross-reactive. This compromise on specificity has profound implications because the chance of any natural peptide-MHC ligand being an optimal fit for its cognate TCR is small, as there will almost always be more-potent agonists. Furthermore, any TCR raised against a specific peptide-MHC complex in vivo can only be the best available solution from the naive T cell pool and is unlikely to be the best possible solution from the substantially greater number of TCRs that could theoretically be produced. This 'systems view' of TCR recognition provides a plausible cause for autoimmune disease and substantial scope for multiple therapeutic interventions.