How T Cells Count

The rules by which the T cells of the immune system are activated by foreign peptides have seemed inconsistent and counterintuitive. In this week's issue, Viola et al. (p. 104) report that the T cells actually count how many T cell receptors are engaged. When the number 8000 is reached, the T cell becomes activated. In her Perspective, Rothenberg describes the many questions this discovery resolves.