Mature T and B lymphocytes respond to receptor-delivered signals received during and following activation. These signals regulate the rates of cell death, growth, differentiation and migration that ultimately establish the behaviour patterns collectively referred to as immune regulation. We have been pursuing the philosophy that in vitro systems of lymphocyte stimulation, when analysed quantitatively, help reveal the logical attributes of lymphocyte behaviour. The development of carboxyfluorescein diacetate succinimidyl ester (CFSE) to track division has enabled the variable of division number to be incorporated into these quantitative analyses. Our studies with CFSE have established a fundamental link between differentiation and division number. Isotype switching, expression of T cell cytokines, surface receptor alterations and changes to intracellular signalling components all display independent patterns of change with division number. The stochastic aspects of these changes and the ability of external signals to independently regulate them argue for a probabilistic modelling framework for describing and understanding immune regulation.