Healthy mind, healthy body? The relationship between the brain and immune system

Up until a few decades ago, it was thought that the brain and the immune system existed as separate entities. Now, we know that nothing could be further from the truth.

Dr Adam Walker has recently joined NeuRA as a Research Fellow and Head of the Laboratory of Immunopsychiatry. He is exploring the intricate relationship between the immune system and the brain, and the role it plays in the development of psychiatric disorders.

Dr Walker says there are many pathways through which the immune system talks to the brain.

“These include nerves, blood, lymphatics and active transporters across the blood-brain-barrier, which is the gatekeeper between the blood and the brain.

“This communication is important for regulating many adaptive aspects of brain function.

“For example, when you have an infection like the flu, your brain needs to know so that it can induce fever and direct you to behave sick,” says Dr Walker.

That’s right – when you’re sick and you sleep more, feel down, lose your appetite – these behaviours aren’t accidental. They are adaptive behaviours directed by your brain in response to immune signalling. They help us rest and recover, conserve energy, and limit the spread of infection.

However, Dr Walker says that there are cases when the immune system can hijack the brain and cause long-term mental health issues.

“A switch can happen in the brain where someone transitions from adaptive ‘sickness behaviours’ into chronic, depressive symptoms, even if the initial infection and inflammation has subsided,” says Dr Walker.

“At NeuRA, we’re trying to identify what causes this transition and determine how we might prevent and treat inflammation-induced psychiatric illness.”

The brain also talks back to the immune system, and under conditions of chronic stress, can impair its ability to fight disease while exacerbating inflammation.

These systems were designed early in the evolutionary process for effective immune-brain communication, but our biology hasn’t necessarily caught up with our social evolution, says Dr Walker.

“We are now faced with daily family and work stress that in some cases, the system wasn’t built for.”

For example, imagine a scenario where you’re dealing with chronic stress at work. This causes chronic activation of our biological stress systems – the sympathetic nervous system ‘fight or flight’ response, and/or our hypothalamic pituitary axis cortisol response.

This over-activation and chronic stress can elevate inflammation in your brain, impeding its optimal function and making you less resilient. It can also suppress your immune system, making you more susceptible to catching the office cold.

Maybe you have a runny nose at first, but you just can’t seem to shake it off. You end up taking a week off work, and the cycle continues until you discover that you’re feeling down all the time and find little joy in the things you used to. You don’t interact with your friends and family and are feeling depressed.

Of course, this is an extreme example. Most people who get the flu don’t become depressed. And being stressed doesn’t always lead to poor immune health.

The good news is that you can promote healthy mental and physical wellness at home and at work by managing stress, exercising, and getting good quality sleep.

For those people who don’t respond to these healthy behavioural preventions, or may already suffer from inflammation-induced psychiatric illness, Dr Walker is investigating how to repurpose currently available immune-targeted therapies and design novel drugs that can treat inflammation-induced mental illness.

 

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