Quantitative rules for lymphocyte regulation: the cellular calculus and decisions between tolerance and activation.

The innovation of fluorescent division-tracking techniques has elevated our understanding of lymphocytes to a new level. These techniques applied in vitro have identified quantitative rules for lymphocyte differentiation, proliferation and survival that had previously been hidden. The many patterns of quantitative response revealed by these analyses provide a sharp contrast to the traditional idea that T cells must make a binary choice between tolerance and activation. Here, evidence for the classic dogma of two-signal theory and T-T help is re-examined in the light of the new quantitative view to show how logical difficulties can be resolved.