Since the discovery of T cells that express two T cell receptors (TCRs), termed dual TCR cells, most studies have focused on their autoimmune potential, while their beneficial roles remained elusive. We identified, in normal mice, dual TCR cells that participated in the immune response to a foreign antigen. Unlike single TCR cells, dual TCR cells used the nonselected TCR to respond in the periphery, but relied on coexpression of a second TCR for intrathymic selection. We found that they were selected at low frequency in the naïve repertoire, but dominated the response to antigen through clonal expansion. Thus, dual TCR cells can extend the TCR repertoire for foreign antigens by rescuing functional TCRs that cannot be selected on single TCR cells; they can, therefore, benefit the immune system.