T cell activation
In most cases, antigen
must be "shown" to the t cell receptor along with a protein
If the cell with the antigen/mhc also has the proper co-stimulatory proteins, the t cell is activated (like needing 2 keys to fire a missle). If the extra proteins are not present, the t cell may cease to function. If the t cell was already activated, it can produce chemicals and/or kill the other cell.