Signal 1
Once cell adhesion has been achieved, antigen presentation can begin.
The antigen/MHC binding to the TCR provides the T cell with its first signal for activation - Signal 1.
Signal 1 is not initiated however, until this process occurs at many TCRs, this is done by 'serial triggering'.
MHC/TCR interactions are very weak and have a high on/off rate, meaning one MHC complex man serially trigger hundreds of TCR molecules. This increases the efficiency of antigen presentation, especially for rare MHC/peptide complexes.
100-1000s of TCR need to be triggered to activate Signal 1.
Signal 2
along with TCRs, and CD8/4 molecules, the T cell has CD28 molecules that will bind to CD80 on the DC, however CD80 is only expressed in the presence of a pathogen.
When both Signal 1 and Signal 2 are achieved, T cell activation and proliferation is achieved.
