Final answer:
The ligands for TCR binding sites are always processed antigenic fragments associated with MHC molecules. There are two types, with MHC I molecules presenting to cytotoxic T cells and MHC II molecules presenting to helper T cells.
Step-by-step explanation:
The ligands for TCR (T-cell receptor) binding sites are always processed antigenic fragments presented in the context of Major Histocompatibility Complex (MHC) molecules. TCRs are present on the surface of T cells and recognize antigens that are processed by antigen-presenting cells and then presented on the cell's surface in combination with MHC molecules. There are two types of MHC molecules: Class I and Class II. Class I MHC molecules present antigens to cytotoxic T cells, while Class II MHC molecules present antigens to helper T cells. Therefore, the ligands for TCR on helper T cells are antigens presented with MHC II molecules, while those for cytotoxic T cells are associated with MHC I molecules.