T lymphocytes are cells of the immune system that attack and destroy virus-infected cells, tumor cells and cells from transplanted organs. This occurs because each T cell is endowed with a highly specific receptor that can bind to an antigen present at the surface of another cell. The T cell receptor binds to a complex formed by a surface protein named "MHC" (major histocompatibility complex) and a small peptide of about 9 amino-acids, which is located in a groove of the MHC molecule. This peptide can originate from a protein that remains within the cell (Fig. 1). Whereas each T cell recognizes a single antigen, collectively the T cells are endowed with a large diversity of receptors targeted at a wide variety of antigens. T cells originate in the thymus. There a process named central tole