T-cell activation: the power of one.

Adaptive immunity depends on antigen-specific activation of resting lymphocytes. Using high-resolution live-cell imaging, a single ligand has been found to trigger a biochemical response in T cells. On the basis of this and other recent findings, a 'pseudodimer' with one foreign- and one self-antigen-engaged receptor linked via a CD4 molecule has been proposed as the fundamental unit of effective T-cell signaling.