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A substantial fraction of organisms that have ever lived have probably died as a result of predation, and much of evolution and of the structures of organisms can be viewed in the context of the battle to eat or be eaten. And as life would have it, many organisms alive today survive only by consuming other forms of life. Predation, broadly defined as obtaining nutrition from other living organisms, is a primary way in which organisms secure their energy needs. As a consequence, most animals and plants are meals-on-a-platter for some other organism(s). Not being consumed by somebody else before reproducing is a primary requisite for darwinian fitness.

Avoidance of predation by large organisms has clearly been important, and the living world has evolved many mechanisms of detection, defense, and escape. However, probably the first, and certainly still among the most dangerous, forms of predation is attack by microorganisms such as bacteria, viruses, and other parasites. These organisms are too small to be perceived by the cognitive or "organismal" sensory systems by which larger predators are perceived. Micropredators enter the body, where they can reproduce and grow and attack their prey from within. Typically they can reproduce rapidly, becoming in a sense an internal megapredator. There thus ensues a race against the attack.

Not only are microorganisms small, but they frequently attack through molecular rather than physical means, by recognizing and binding to various cell surface molecules within the prey and entering the cells, or even insinuating themselves into the host's DNA. But if their small size makes their presence difficult to detect cog-nitively by the host, their size means that they can be defended against by molecular means. This is convenient for complex organisms whose life is itself based on means to interact by molecular signaling and recognition. Defense against microorganisms can do double duty as defense against nonreplicating but potentially threatening exogenous microscopic or molecular entrants into the body—toxins, pollen, foreign protein, and the like. Even if not alive, such agents can be dangerous if exposure is extensive or extended over time.

Genetics and the Logic of Evolution, by Kenneth M. Weiss and Anne V. Buchanan. ISBN 0-471-23805-8 Copyright © 2004 John Wiley & Sons, Inc.

All multicellular organisms have evolved mechanisms for protecting against molecular attack. Given that the number of pathogens and their ability to evolve rapidly greatly outstrips that of most of their prey, self-defense is no small task. It would seem, on the face of it, an almost impossible battle for big, long-living (and sometimes stationary) organisms to outcompete the swift and nimble microparasite attack. Yet this obviously happens.

The challenge in molecular self-defense is not just a matter of recognizing alien organisms within. In some way or another, an organism fighting molecules with molecules must obey the dictum "above all, know thyself." That is, organisms must be able to distinguish their own cells from pathogens, and to preferentially target the pathogens for destruction, a daunting requirement. Yet complex organisms are themselves molecularly very diverse and, due to somatic mutation and cellular differentiation, changeable during their lifetimes. Some organisms have very complex mechanisms to deal with these challenges, but others get along very well with simple mechanisms.

Some aspects of immune defenses are shared between extremely divergent organisms. In this chapter, we will discuss the immune systems of insects, plants, and animals and what is currently thought about how they evolved. Because it includes most of the components found in the immune systems of other organisms, we will begin with the most complex system, that found in vertebrates. In addition, although at first blush the necessity for an immune system at all seems to confirm the idea that life is primarily a battle to the death between competing organisms, in fact, many if not the most successful pathogens do not kill their hosts but instead have evolved a coexistence; most organisms peacefully serve as hosts to many microorganisms, and indeed cannot do without them. The challenge to live a completely aseptic life might be—or at least has proven to be—beyond the capacity or the interest of evolution.

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