When a cytotoxic T cell (CTL) recognizes a peptide presented in the MHC-1 of a dendritic cell (APC), why it doesn't kill this cell?
I know that initially, in the lymph node, the T cell is inactivated. But eventually it becomes activated and travels to the tissue, where it again finds APCs presenting the offending antigen. So why doesn't the CTL kill the APC at this point?
Moreover, not all MHC-1's of the dendritic cell present antigens from phagocytized cells. Some MHC-1's present peptides coming from the dendritic cell itself, so that if the dendritic cell itself becomes infected with virus or bacteria, it can be killed by a CTL that recognizes the peptide. So in this case, the CTL does indeed kill the dendritic cell.
So my question is, when exactly does the CTL kill the APC? How does the CTL knows that should or shouldn't kill the APC?