  The adaptive immune system responds to foreign invaders by activating T
cells. However, this response is perilous if T cells are activated by the wrong
signal or if they remain activated unduly long after the threat has been
eliminated. It is therefore important that T cells get activated only by the
right kind of signals and for the right duration. The dominant theory in
immunology over recent decades has been that a T cell must receive at least two
signals before it can become activated. It is, however, unclear whether and how
this two-signal requirement ensures that T cell activation is provoked only by
the right signals and that the response is just long enough. Here, we propose
that the two signal requirement induces a novel coherent feedforward motif
whose properties align with those preferred for an ideal immune response.
Further consideration of the interaction between helper and regulatory T cells
induces a composite feedforward-feedback motif, analysis of which generates a
condition for a healthy concentration of activated T helper cells.
