This is a CD8 T-cell, a special kind of immune cell that can target and destroy tumor cells (cell nucleus is blue, and the cell membrane is green). It has been labelled with these nanoparticles (red rings) with special optical properties – that we can see them through tissue! With the right kind of laser and detector (that we have in our lab) we can see these nanoparticle-loaded cells travelling through the body in real time to better understand anti-tumor response, which is highly variable from person to person and affects outcomes to cancer therapies.