The art of the probable: system control in the adaptive immune system.

The immune system provides very effective host defense against infectious agents. Although many details are known about the cells and molecules involved, a broader "systems engineering" view of this complex system is just beginning to emerge. Here the argument is put forward that stochastic events, potent amplification mechanisms, feedback controls, and heterogeneity arising from spatially dispersed cell interactions give rise to many of the gross properties of the immune system. A better appreciation of these underlying features will not only add to our basic understanding of how immunity develops or goes awry, but also illuminate new directions for manipulating the system in prophylactic and therapeutic settings.