T cells are central to many aspects of adaptive immunity. Each mature T cell expresses a unique αβ T cell receptor (TCR) that has been selected for its ability to bind to peptides presented by major histocompatibility complex (MHC) molecules. Unlike antibodies, TCRs generally have low affinity for ligands (KD ~ 1-100 μM), which has been speculated to facilitate rapid scanning of peptide-MHC (pMHC).