On a population level, T cell activation depends on the strength of TCR stimulation, the dose–response curve of ligand to cytokine production is sigmoidal. However, T cell activation on a single cell level can be characterised by a digital switch-like response, meaning the T cell is fully activated if the stimulus is higher than a given threshold, otherwise the T cell stay in its non-activated state. There is no intermediate activation state. The robust sigmoid dose-response curve on population level results from individual T cells having slightly different thresholds.