Fuld aktivering af T- lymfocytter kræver 2 signaler fra celler med antigener: genkendelse af et specifikt antigen af en T- cellereceptor (signal 1) og et andet co- stimulatorisk signal.
Full activation of T lymphocytes requires two signals provided by antigen presenting cells: recognition of a specific antigen by a T cell receptor (signal 1) and a second, costimulatory signal.