For every invading pathogen there will be a worst- case scenario as to what kind of molecules it might
encounter. Our immune systems have been shaped over a hundred million years to make the pathogen’s worst nightmares come true. Unfortunately, every effective weapon can sometimes be dangerous to the one who wields it.
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The immune system can make two kinds of mistakes: failing to attack when it should and attacking something when it shouldn’t. The first kind of mistake results from inadequate response, so that a disease that should have been nipped in the bud becomes serious. The second kind of mistake results from mounting too aggressive a response to minute chemical differences. Autoimmune diseases such as lupus ery- thematosus and rheumatoid arthritis could be the result. The average person’s degree of sensitivity and responsiveness is presumably close to what has historically been the optimum: enough to counter pathogens but not so great as to attack the body’s own structure.
Given that we have this chemical superweapon-immunity-how can we possibly remain vulnerable to infectious diseases? Once again, it is because the infectious agents can evolve rapidly and become bet- ter adapted by natural selection. Those variants that are least vulner- able to immunological attack will be those whose genes are best represented in future generations. So the pathogens may evolve one or another kind of defensive superweapon. Molecular mimicry, men- tioned in the last chapter, is one such weapon.