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 re- jections, different
molecular structures responsible for intercellular interactions such as
interleukins, cytokins, idiotype-antiidiotype recognition and others.
While furthering our understanding of the local interactions (molecular
and cellular) in- volved in the immune response, these discoveries have
led to a questioning of the simplicities of the classical clonal
selection theory. It is clear today that every single immune response is
a cooperative phenomenon involving several different molecular and
cellular interactions taking place in a coupled manner. In addition,
cross reactivity to different antigens has shown that responses of the
whole im- mune system to different antigens are not completely isolated
from one another and that the history of encounters with different
antigens plays a crucial role in the maturation of the whole system.
Thus, problems of complexity, generation of di- versity and
self-organization have entered the field of immunology.