When we get sick, we often feel tired, moody, or anxious.
Scientists have long known that this isn’t just about feeling weak—it also involves how the brain is affected by signals from the immune system.
A new set of studies from researchers at MIT and Harvard Medical School explains more about how this connection works.
They found that an immune molecule called IL-17 not only helps fight infections but also affects how we feel and behave.
IL-17 is part of a group of molecules called cytokines. These are made by the immune system and are important for defending the body against infections.
Cytokines help control inflammation and tell other immune cells what to do. But now it’s becoming clear that they also influence the brain, changing our emotions and behavior.
The researchers discovered that IL-17 affects two different parts of the brain. One part is the amygdala, which is involved in emotions like fear and anxiety. The other part is a small section of the cortex, which helps control social behavior.
When IL-17 acts on the amygdala, it makes animals more anxious. But when it acts on the cortex, it makes them more sociable. This shows that the immune system and the brain are closely connected.
One of the lead scientists, Professor Gloria Choi from MIT, says that being sick doesn’t just make you physically tired. It also changes how your brain works and how you behave. That’s why people often feel different emotionally when they’re ill.
Years ago, Choi and her collaborator Professor Jun Huh from Harvard became interested in IL-17 when they studied something called the “fever effect.” Some children with autism show fewer symptoms when they have a fever. In a 2019 study in mice, the team showed that IL-17 may be involved in this effect.
They found that IL-17 could calm down a small area of the brain called S1DZ. This area, when too active, can lead to behaviors in mice that resemble autism, such as repetitive actions and avoiding social interaction.
In their new studies, the team looked more closely at how IL-17 works in the brain. They discovered that different forms of IL-17 attach to different receptors in different brain areas. In the cortex, one type of IL-17, called IL-17E, connects to receptors that calm certain neurons. This helps improve social behavior in mice.
Interestingly, the brain itself can make IL-17E. This suggests that this molecule might have first evolved to work in the brain, and was only later used by the immune system to fight infections. In simpler animals like worms, IL-17 doesn’t help fight infections at all. Instead, it helps control social behavior.
In another part of the study, the researchers looked at the amygdala. They found that when another form of IL-17, like IL-17A or IL-17C, acts on this region, it makes the neurons more active and causes more anxiety. This may be helpful when someone is sick, because feeling anxious can make them stay away from others and avoid spreading illness.
However, there could be a downside. In some cases, blocking IL-17 in the body actually increases the amount of IL-17C, which might explain why a psoriasis treatment that targeted IL-17 caused mental health side effects in some people. The scientists think this rise in IL-17C may have affected patients’ brains and even led to suicidal thoughts in rare cases.
The researchers also found that the same brain cells that respond to IL-17 also have receptors for IL-10, a molecule that calms inflammation. IL-10 can reverse the anxiety caused by IL-17, which helps the brain return to normal once the body is no longer sick.
Overall, these studies show that immune molecules like IL-17 play a powerful role not only in protecting the body but also in shaping how we think and feel. The same molecule can have very different effects depending on where it acts in the brain. One version can make us anxious, while another can make us more social.
These findings could one day lead to new ways to treat brain-related conditions like autism or depression. Instead of targeting the brain directly, scientists might be able to use the immune system to improve mental health.
The research also highlights just how connected our bodies and minds really are—our thoughts and feelings are shaped not just by what’s happening in the brain, but also by what’s happening in the immune system.
If you care about health, please read studies that scientists find a core feature of depression and this metal in the brain strongly linked to depression.
For more information about health, please see recent studies about drug for mental health that may harm the brain, and results showing this therapy more effective than ketamine in treating severe depression.
The research findings can be found in Cell.
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