How the Immune System Interacts with Our Brains

Post by Anastasia Sares

What did we learn?

Neuroscientists are learning more and more about the role of the body’s immune system, the gut microbiome, and physical illnesses on brain function. Immune systems, when they are working well, defend the body and brain against infection. The brain has its own immune system, which normally stays separated from the rest of the body, protected by a layer of defense called the blood-brain barrier. While much of neuroscience focuses on neurons and the web-like connections between them, support cells (astrocytes) and immune cells (glia) can drastically affect the function of neurons. And if the blood-brain barrier is compromised, both diseases and the body’s immune cells can cross over and create complications.

In many dementias, astrocytes and glia can go rogue and attack neural synapses. In Alzheimer’s disease for example, abnormal proteins may use the cholesterol transport mechanisms in astrocytes to spread to different parts of the brain. Immune cells that are normally outside of the brain can also make their way into the brain when the body is under stress. In Lewy-Body Dementia, immune cells that are normally only found in the body appear around the sites of damaged brain tissue, and their immune response is amped-up. In addition, immune responses inside and outside of the brain can lead to neural inflammation, which may be one of the mechanisms of depression.

Still more surprising connections can be found between digestive health, including the gut microbiome, and neural health. A study by Wu and colleagues found that mice bred without microbiomes in their gut display reduced social behavior towards other mice, and bigger stress response to the few social encounters they did have. In fact, it has recently been suggested that during pregnancy, maternal infections and their corresponding immune responses may lead to both Autism Spectrum Disorder and digestion problems in the fetus, which could be why Autism and digestion issues are often linked. This is part of a research trend looking at the gut-brain connection, which made its way into pop science starting in 2015 with books like Gut, The Brain-Gut Connection, and the Psychobiotic Revolution.

Then there are, of course, the neurological side effects of the Sars-Cov-2 virus, which emerged in 2019 and has been the focus of fever-pace research since then. For example, a post-mortem study compared brains of people who died from COVID-19 to brains of people who died from other causes, and found that COVID patients had increased brain inflammation and damage to axons in the brain (the tendrils that neurons use to communicate with one another). The fact that COVID can cross the blood-brain barrier and have long-lasting effects on the nervous system make vaccination access and uptake even more important.

What's next?

Though a good proportion of studies examining the connection between the nervous system and the immune system are still taking place in mice and rats, there seems to be a transition toward more human studies, solidifying the connection between these two systems. Looking at the human body as a whole is bound to bring new insights into how our immune systems interact with and impact brain function.