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Inflammation, infection and depression: an evolutionary perspective
- Caroline Doyle, Walker A. Swain, Holly A. Swain Ewald, Paul W. Ewald
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- Journal:
- Evolutionary Human Sciences / Volume 1 / 2019
- Published online by Cambridge University Press:
- 09 December 2019, e14
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- Article
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The evolutionary basis for clinical depression is not well understood. A growing body of literature that is not based on evolutionary logic links inflammation to depression. Integration of these findings with an evolutionary framework for depression, however, needs to address the reasons why the body's inflammatory response would be regulated so poorly that it would result in incapacitating depression. Pathogen induction of inflammation offers an explanation, but the extent to which the association between inflammation and depression can be attributed to general inflammation as opposed to particular effects of pro-inflammatory pathogens remains unclear. This paper reports a study of sexually transmitted pathogens, which addresses this issue. Although several sexually transmitted pathogens were associated with depression according to bivariate tests, only Chlamydia trachomatis and Trichomonas vaginalis were significantly associated with depression by a multivariate analysis that accounted for correlations among the pathogens. This finding is consistent with the hypothesis that infection may contribute to depression through induction of tryptophan restriction, and a consequent depletion of serotonin. It reinforces the idea that some depression may be caused by specific pathogens in specific evolutionary arms races with their human host.
7 - Evolutionary insights for immunological interventions
- from Part III - Evolution and Medicine
- Edited by Aldo Poiani, Monash University, Victoria
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- Book:
- Pragmatic Evolution
- Published online:
- 05 April 2012
- Print publication:
- 10 November 2011, pp 115-132
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Summary
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.