Schematic Representation of the TCR Affinity Thresholds Involved in Antigen-Driven Selection.
Sorting helper T cells according to the ability of their T cell receptors to recognize antigen reveals that there are affinity thresholds that determine whether they will be selected for clonal expansion. Helper T cells with too low affinity don't initiate a proliferative response upon antigen encounter and do not cross the first threshold. Similarly, some helper T cells make an initial proliferative response up to day three but then fail to propagate further because they do not have high enough affinities to cross the second threshold.
However, all helper T cells that do cross the second threshold undergo identical clonal expansion to day seven. Those with the highest affinities for the antigen have no advantage.