The Other Side of the Arms Race

Abstract

Having read Chaps. 3 and 4, one might well be forgiven for assuming that pathogens should have long since ceased to be a problem. After all, immune receptor repertoires have been selected to “see” all sorts of different pathogens, and the information they generate can activate powerful effector mechanisms that should certainly be able to destroy all intruders. But we all know that this is not the case, and the reason is that, host–pathogen interactions are always an arms race (see Sect. 1.9), and so any effective defensive move made by the host merely provides the selection pressure that drives the evolution of new virulence strategies in pathogens. And indeed, random mutation and selection have provided pathogens with all sorts of ways to evade immune system receptors, to interfere with the subsequent signal transduction pathways, and to escape from terminal effector systems such as complement, granulocytes and CD8⁺ T-killer cells. In its turn, every new pathogen virulence strategy drives the selection of new host resistance mechanisms. It is a never-ending struggle in which sometimes the pathogen, and then sometimes the host, manages to achieve a short-lived advantage.