“The Acquired Immune System: A Vantage from Beneath”, 2004-11-01 (; backlinks; similar):
The immunity exhibited by plants and animals is often viewed as the evolutionary response to the problem of infectious agents. In this respect, the combination of the innate immune system and the acquired immune system has been characterized as the “optimal solution.”
In this essay, I propose that there is no possibility of an optimal solution to the problem of parasitism. Regardless of the immunological mechanisms evolved, infectious agents establish a dynamic interaction with common strains of their host species, weighing virulence against transmissibility.
In the endless host-parasite coevolution, the immune system can never gain an upper hand on the millions of parasitic microbes and viruses. Rather, evolution of the immune system is driven, most importantly, by the small advantages conferred as a result of host variation.
By selecting for ever-more-devious parasites, the immune system is the cause of its own necessity.