An intracellular pathway that initiates the process of programmed cell death (apoptosis). Fas is a cell surface receptor: when it binds its specific extracellular signal molecule, Fas ligand, the intracellular portion of the Fas protein changes shape, enabling it to bind adaptor proteins and recruit components of apoptosis, notably caspase enzymes, which act as ‘executioners’ inside the cell. Some effector T cells express Fas ligand on their surface and can kill target cells by binding to Fas and triggering apoptosis. Moreover, after an infection has been cleared, redundant lymphocytes are induced to enter apoptosis by a similar mechanism.