Most complex biological systems, such as enzyme pathways, are effec tively controlled near the beginning of the process. There is increasing evidence that the same is true for the immune system, with the initial interactions between antigen, antigen-presenting cells, and T cells hav ing a paramount influence on the ensuing events. Thus, analysis of the early stages of the immune responses has been a preoccupation of many immunologists. This has been considerably aided by the capac ity to expand these early events, and 'immortalize' them as clones of T cells, for detailed analysis. The...