On the evolutionary constraints necessary for immunity

Abstract

All organisms represent a tempting storehouse of resources for other organisms to exploit. This fact leads to the evolution of semi-permeable defensive boundaries. Nutrients must be allowed in, and waste products allowed out. Aggressors should be kept out if possible or identified
and attacked once they breach the boundary. An ongoing co-evolutionary battle leads, on the defensive side, to the finely tuned mechanisms of the immune system.

The thesis takes a functional perspective on immune system evolution. We begin with a historical review of the scientific understanding of immunity, and then ask what features of the immune systems we see today are general enough to occur again if we replayed the tape of life on Earth, and what features are likely to be merely contingent. There are computationally useful processes
found in immune systems, but separating these transcendental principles from observable biological implementations is difficult.

If pathogens had infinite flexibility to change their appearance, or if hosts had similar flexibility to alter their metabolism, the co-evolutionary battle between them could not occur. We therefore argue that an evolutionary constraint necessary for immune system evolution is the concept of entrenchment or ‘lock-in’. We use a variation on the NK-landscape model to show how the structure of the dependency network between traits affects both evolvability and entrenchment and thus promotes the evolution of immune systems.

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