What makes a complex system vulnerable ?

Needless to say, the immune system is both complex and vulnerable. Why makes a simple system switches to a complex one is: the non-linearity of the interactions, the non-homogeneity of the interaction topology (some biological nodes are much more connected than others) and, above all, the gradual substitution of a linear causality by a circular one (in the presence of many positive and negative feedback loops). This latter reinforces the behavioral autonomy of these systems (as compared with the importance given to external impacts) and gives rise to the most intricate dynamical regimes such as multi-stability, cycles or chaos. Those well-known insights coming from physics and computer sciences allow shedding new lights on these systems vulnerability: the self-organization of a homeostatic/viable regime, the possibility to autonomously getting closer to bifurcation frontiers and the elimination of strategic nodes. The talk will remind those classical physical facts and how, for more than 30 years, they considerably influence how my inspirers (Coutinho, Varela, Stewart) and myself perceive immune