The T cell antigen receptor (TCR) complex with CD3 consists of six polypeptide chains: TCRα, TCRβ, CD3δ, CD3ε, CD3γ and Cd3ζ. Only 1-5% of newly synthesized TCRα and TCRβ chains reach the T cell surface. This is due to complicated intracellular protein interactions at several crucial checkpoints. More than 90% of TCR chains are degraded in the cytosol, processed by the proteasomes and rendered available for protein assembly with major histocompatibility complex (MHC) class I molecules. Consequently, TCR are present on the T cell surface in two forms, as antigen receptors and as peptides associated with MHC class I molecules, i.e. ligands for CD8+ T lymphocytes. In the present review, we summarize our data on how the two discrete functions of TCR molecules may function in the regulation of autoimmunity and tumor cell rejection.