Offensive and defensive strategies against evolving threats

Infectious and parasitic diseases have been paramount among the threats to human health and survival for most of our (and our mammalian ancestors') evolutionary history. These diseases present three particular evolutionary problems. First is the essentially open-ended variety of pathogens to which we might be exposed. Secondly, while being able to attack nearly any kind of microorganism, we must be inhibited from attacking our own, very diverse, cells. Thirdly, we reproduce and can evolve genetic adaptations only slowly, but the pathogens that infect us typically reproduce in the millions, in days or even hours. These seem like evolutionary battles we could hardly win.

One way the immune system recognizes foreign pathogens is by their molecular structure; but in contrast to molecular recognition systems such as those between hormones and their receptors, which can be specifically programmed, the immune system has evolved to use a mechanism that can recognize an unlimited diversity of molecules {Roitt et al., 1989}. That mechanism forces us to revise the standard assumption that the inherited genome is constant during the life of an individual.