Figure 1.

The relationship between tolerance and discrimination becomes more complex when negative selection is incomplete. If negative selection were “complete”, all self peptides would be presented in the thymus and all self-reactive T cells would be silenced (case 1). In other words, all self peptides would be completely tolerated (no responding T cells left, gray area), and there would be perfect self-foreign discrimination (dashed region: all peptides that are still properly recognized are foreign). The only way to have no discrimination in this scenario is if negative selection would be “too complete”, such that not only all self peptides, but also all foreign peptides are completely tolerated (case 2). If negative selection is incomplete, low tolerance can occur with either very strong (case 3), or very low discrimination (case 4).