For a long time, immunology has been dominated by the idea of a simple linear cause-effect relationship between the exposure to an antigen and the production of specific antibodies against that antigen. Clonal selection was the name of the theory based on this idea and it has provided the main concepts to account for the known features of the immune response. More recently, immunologists have discovered a wealth of new facts, in the form of different regulatory cells (helpers, suppressors, antigen presenting cells), genetic determinations of immune responses such as those involved in graft...