An evolutionary perspective reveals why immune processes are intricately interconnected with each other and with other biological processes within multicellular organisms. This web-like interconnectedness has important implications for medical interventions. Evolutionary considerations suggest that direct manipulation of molecules and processes within the immune system are inevitably going to be associated with detrimental side-effects that result from the web-like interconnectedness. Manipulations of the immune system's exposure to threats, however, has led to many of the most successful achievements of medical science, because this sort of manipulation takes advantage of the inherent strengths of an intact immune system, by giving the immune system the upper hand in its attempts to control these threats. Evolutionary considerations also offer a new way to complement the intact operation of immune systems – by designing health interventions, such as vaccination strategies, to control the evolution of pathogen virulence.

Evolution of immune systems

One of the problems the first multicellular organisms encountered was the need for protection against unicellular organisms (e.g. bacteria) or subcellular agents (e.g. viruses). Although the mechanisms of this parasitism were probably similar to those involved in parasitism of unicellular hosts, multicellular organisation posed some additional vulnerabilities. Multicellular organisms required time to develop from a single cell and evolved cellular specialisation for different tasks. The longer time associated with multicellular development created a vulnerability, because any parasite that could circumvent the defences of a single cell could have immediate potential access to the host's other genetically identical cells. To restructure the genetic basis for its defences, the multicellular host would have to await development to maturity when sexual reproduction could create genetically different cells (Hamilton, 1980). A host's cells might still be able to rely on the defences that unicellular hosts have against parasites, such as destruction within phagosomes, but competition between multicellular organisms favours specialisation of cells for different purposes, such as neuronal conduction, support, sensing stimuli, proliferation and reproduction. Maintaining the entire suite of defensive armaments within each cell would compromise the ability of cells to be superior at their specialised functions.