Significance T cells recognize their targets through the T cell receptor (TCR). The affinity of a typical receptor for an agonist peptide-major histocompatibility complex (pMHC) molecule is extremely weak, and TCRs are known to be cross-reactive for related peptides. However, there are known TCR/pMHC interactions that occur at weaker affinities, such as in thymic selection and recognition of self-antigens, yet little is known about the identity of these peptides. We show that TCR/pMHC interactions of extremely low affinities remain highly specific, which informs of the nature of extremely weak affinity ligands. We also show that a peptide “velcro” can induce peptide-dependent T cell activation, providing a method for increasing the potency of a target, which is useful in immunotherapy.