Our immune system consists of a complex biological network that is designed to protect us against both harmful infections and internal damage. However, in recent decades evidence has begun to emerge that shows how our immune system is also linked to our mental health.

Higher rates of depression and fatigue are found across a broad range of conditions associated with excessive activation of the immune system, including allergies, autoimmune diseases (such as type I diabetes, multiple sclerosis and rheumatoid arthritis) and infections^[1]. Similarly, people who have been subjected to neglect or childhood abuse are not only more susceptible to mental health issues in adulthood, but are also at a higher risk of disorders related to the immune system^1,[2]. Recent data even shows that there is elevated immune activation in mothers with post-natal depression lasting several years after delivery^[3].

While these associations have been observed for some time, we are now beginning to gain insight into the mechanisms that might be responsible for this link. Large cohort studies have shown that not only are there increased proinflammatory cytokines (signalling molecules that promote inflammation) in people with depression, but also that children with high levels of these molecules are more at risk of depression in adulthood^1,2. Increased inflammation caused by these molecules may disrupt the barrier between the body and brain (blood brain barrier), allowing molecules to enter the brain and disrupt normal signalling^1,2.

Whilst clinical studies have provided mixed results so far, there is now evidence to suggest that the link between inflammation and mental health could be important for subsets of patients who respond poorly to antidepressant treatment. For example, at present there are no reliable predictors of how individuals will respond to antidepressants. However, it is possible that immune markers may be used to predict treatment efficacy of some antidepressants¹. Furthermore, research is now underway into the efficacy of using anti-inflammatory drugs, such as steroids, in treating depression^1,2.

The way that we diagnose and treat mental health hasn’t changed much in decades^[4]. Therefore, understanding the link between the immune system and mental health could be part of the answer to unlocking new, more effective, and targeted treatments.

[1] https://www.frontiersin.org/articles/10.3389/fimmu.2019.01696/full

[2] https://www.cell.com/neuron/fulltext/S0896-6273(20)30431-1

[3] https://onlinelibrary.wiley.com/doi/10.1111/aji.13619

[4] https://wellcome.org/news/why-we-need-better-markers-anxiety-and-depression

Edited by Hazel Imrie

Copy-edited by Rachel Shannon