Et væsentligt co- stimulatorisk led involverer binding af CD80 - og CD86- molekyler på overfladen af celler med antigener for CD28- receptoren på T- lymfocytter (signal 2).
A major costimulatory pathway involves the binding of CD80 and CD86 molecules on the surface of antigen presenting cells to the CD28 receptor on T lymphocytes (signal 2).