COSTS AND BENEFITS OF THE IMMUNE RESPONSE

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.

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.