Illustration of gold nanoparticles interacting with a T cell. T cells, or T lymphocytes, are components of the body's immune system. Helper T cells stimulate other immune cells to act against a pathogen, whereas killer T cells target and destroy infected cells themselves. Gold nanoparticles have a number of properties that make them useful in living systems. They are non-toxic, small enough to enter cells, and can carry a variety of therapeutic and diagnostic agents.