To arrive at a deeper understanding of complex adaptive systems?to understand what makes them complex and what makes them adaptive?it is useful to look at a particular system. Consider the immune system. It consists of large numbers of highly mobile units, called antibodies, that continually repel or destroy an ever-changing cast of invaders (bacteria and biochemicals), called antigens. Because the invaders come in an almost infinite variety of forms, the immune system cannot simply develop a list of all possible invaders. Even if it could take the time to do so, there is simply not room enough to store all that information. Instead, the immune system must change or adapt ("fit to") its antibodies as new invaders appear. It is this ability to adapt that has made these systems so hard to simulate.
