T-cell activation is of central importance to the generation of an immune response and is also required as part of the host's ability to recognise self proteins. T cells are activated to differing extents by different ligands. Agonist ligands cause the full range of T-cell activation phenotypes – from activation of signalling cascades, to cytokine secretion or target cell killing, to T-cell proliferation. Partial agonists, which can differ from the agonist by as little as a single amino acid residue, can induce some of these responses but not all. Antagonist ligands can disable the signalling of an agonist ligand. These different types of interaction between ligand and T-cell receptor (TCR) also determine the developmental fate of maturing T cells. Much recent work has focused on how the T cell distinguishes between ligands. At least part of the answer lies in the kinetics of its binding to ligand.