Our immune system fights pathogens, repairs wounded tissue and eliminates dead cells. It also acts to ensure we tolerate our own cells, food intake, and other environmental components, as well as the indigenous microbiota.

In the presence of pathogenic invaders and other noxious insults, a resilient immune response is crucial to effectively eliminate the source of stimulation in a way that does not harm the host. Likewise, when the immune system encounters substances or microbes that are not threatening, tolerogenic responses need to be active to avoid inappropriate chronic inflammatory responses.

The bidirectional relationship between resilience and immunity is supported at different levels. Most of the data regarding this connection are based on animal studies, which have reported individual differences in the ability to deal with chronic stress-related conditions.

Mostly correlational human studies have also shown that immune mediators may influence how the brain processes and responds to environmental inputs.

Specifically, inflammation—the major component of innate defense—may adjust host physiology and therefore affect the quality and intensity of the resilience-based response. For instance, activation of brain macrophages and microglia secondary to an infection, gut barrier dysfunction and stress drives the recruitment of CD8-positive T lymphocytes cells into the meninges. This results in immune cells switching to an anti-inflammatory phenotype that facilitates the recovery process. Alternatively, following exposure to mild pathogenic stimuli, the release of pro-inflammatory cytokines may also regulate some aspects of resilience, including positive affect and social support.

These findings all imply that qualities that are part of the resilience process—the most frequently studied elements in stress research are personal control, positive affect, social support, and optimism—may attenuate the adverse effect of stress factors on inflammatory immune responses. Meanwhile, immune mediators may also influence how individuals adapt to and recover from stress.

Gut microbiota functions are important determinants of the host immune response, though the fine-tuned underlying mechanisms remain the subject of ongoing research. In this close interplay between stress, resilience and immunity, stress-induced variations in the gut microbiota can modulate final resilience outcomes, particularly in terms of mental health.

After vaccination, nutrition is considered to be one of the primary exogenous factors that modulate immune function. And taking into account that nutrition particularly impacts the gut microbiota, researchers are exploring to what extent microbiota-targeted nutritional interventions could be used for modulating immune resilience.

This has led scientists to hypothesize that the gut microbiota could be involved in modulating resilience by inducing behavioral and physiological changes. This context has paved the way for the use of diet, and particularly pre- and probiotics, to improve resilience and help prevent mental health-related disorders.

Specific probiotics have been developed to promote positive mental health benefits and psychological resilience (the so-called psychobiotics), with probiotics possibly affecting behavior and brain function in healthy adults.

A further area of interest can be found in the way probiotics (and prebiotics) have helped increase the antigen-specific antibody response to an immune challenge in healthy situations. Research has also highlighted the role of probiotics and prebiotics in regulating how the immune system’s response functions via the gut microbiota, and how, in turn, they influence the immune system. Furthermore, some prebiotics and probiotic strains have been shown to help improve the effectiveness of seasonal influenza vaccinations in elderly people.

We are also awaiting results from large scale placebo-controlled studies before diet, and particularly probiotics, can be used to increase resilience. Besides this, scientists are still struggling to select and validate immune markers that accurately assess how nutrition impacts immune function.

On the whole, both animal and human studies show that the close relationship between the immune response—particularly inflammation—and resilience is partly mediated by the gut microbiome. Further research in humans is needed before particular recommendations are translated into clinical practice.




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