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Feeling sluggish, depressed, anxious, and having trouble concentrating while being sick might not just be due to physical weakness. Extensive reports from health professionals have pointed out that these symptoms, labeled "sickness behavior," are more than just side effects of the body fighting off an infection. They appear to be part of an intentional behavior pattern driven by a collaboration between the brain and the immune system.

This pattern may have evolutionary significance, helping to protect the community by reducing direct contact and preventing the spread of disease. Beyond that, it might hold valuable insights into mental health conditions, potentially leading to new treatment strategies.

Two studies from Harvard Medical School and the Massachusetts Institute of Technology (MIT), published in Cell, have brought us a step closer to understanding the molecular mechanisms behind how inflammation impacts our moods and behaviors. The research has identified which parts of the immune system communicate with the brain to explain this phenomenon.

Cytokines in Brain-Immune Crosstalk

Cytokines are small protein messengers released by a wide range of cells, especially immune cells. They help immune cells communicate, particularly when preparing to launch an inflammatory response to fight off pathogens.

Cytokines are not all the same; they fall into two broad categories: pro-inflammatory and anti-inflammatory, each playing a role in maintaining the delicate balance between fighting infections and preventing excessive inflammation.

To send messages, cytokines bind to receptors on their target immune cells, much like a key fitting into a lock, to fulfill their role in the immune system. Interestingly, cytokines don’t only affect immune responses; their impact extends to the brain, influencing cognition, mood, and behavior.

For example, a 2019 study on mice suggested that IL-17A, a cytokine released during fever caused by inflammatory infections, could explain the temporary reduction in social behavior often observed in autistic children. While this discovery hints at the complexity of brain-immune crosstalk, the full extent of it is still not well understood.

Read More: Visualizing Brain Connectivity May Aide in Diagnosing Mental Illnesses

Uncovering Cytokines’ Role in Mood and Behavior

The same researchers who conducted the cytokine study on autism expanded their research to investigate how and where cytokine signals in the brain affect mood, anxiety, and social behavior.

One study identified how specific cytokines, such as IL-17A and IL-17C, target the amygdala — the brain’s fear center — and influence its neural activity, leading to increased anxiety. Interestingly, when the researchers attempted to block the receptor (receptor antagonism is a common pharmacological effect of many commercial drugs), anxiety actually increased. Another fascinating finding was that the anti-inflammatory cytokine IL-10 seemed to reduce anxiety.

Another study further explored cytokines’ role in mood regulation by returning to autism-like behaviors. It identified several cytokines and their receptors, in addition to IL-17A, that appear to enhance social interactions in mice with autism-like traits. Notably, IL-17E was also found to be produced by neurons in the brain, positioning it as a neuromodulator, much like other neurotransmitters such as serotonin and dopamine.

"Our results emphasize the important role of immune signaling in shaping mood and behavior by acting on specific brain pathways," explained co-senior author Gloria Choi, Associate Professor at MIT’s Picower Institute for Learning and Memory, in a press release.

Potential Alternatives to Traditional Psychiatric Drugs

"By identifying where and how cytokine receptors work in the brain, we have begun to unravel the complex relationship between the nervous and immune systems in the effect of this complex crosstalk on mood and behavior,” said Jun Huh, associate professor of immunology in the Blavatnik Institute at HMS and co-senior author on the two studies in the press release.

While Huh’s and Choi’s research brings us forward in understanding brain-immune communication, more studies are necessary, especially regarding human application.

In the future, new treatments for autism and anxiety disorders could be a result of these initial findings. Unlike traditional psychiatric drugs, which alter brain chemistry directly, these therapies may adjust immune signals from outside the brain, offering a novel approach using the immune system.

Read More: Excessive Worry About Health Could be Signs of Illness Anxiety Disorder

Article Sources

Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:

Having worked as a biomedical research assistant in labs across three countries, Jenny excels at translating complex scientific concepts – ranging from medical breakthroughs and pharmacological discoveries to the latest in nutrition – into engaging, accessible content. Her interests extend to topics such as human evolution, psychology, and quirky animal stories. When she’s not immersed in a popular science book, you’ll find her catching waves or cruising around Vancouver Island on her longboard.

Sweetened vanilla, calming lavender, or fragrant jasmine and lotus may fill your home with enticing aromas. But new research shows that the supposed stress-reducing and mood-enhancing effects of scented products may come with unwanted indoor pollution.

“While these products are widely used to create a cozy atmosphere, their emissions can impact indoor air quality, especially in spaces with limited ventilation,” says Nusrat Jung, a civil engineer at Purdue University.

Jung became interested in the quality of our indoor atmosphere after walking through grocery store aisles that had scented candles, wax melts, and other fragrance-releasing items.

“These products are marketed as safe and clean, but we wanted to investigate what else they might be releasing into the air besides pleasant scents,” she says.

Scented Wax Melts and Pollution

In research published recently in Environmental Science & Technology Letters, Jung and her colleagues examined the effects of scented wax melts that are often advertised as pollution-free. They used a laboratory recreation of a typical home at Purdue filled with sensors that could monitor the kinds of chemicals inside.

Scented products have released volatile organic compounds and terpenes — chemicals responsible for everything from aromatic essential oils to the skunk-like smell of marijuana. But previous research by Jung and her colleagues revealed that flame-free candles, or wax melts, release more terpenes than candles with flames.

Once released, terpenes react with ozone in the air and form nanoparticles.

“These particles, despite being formed in a non-combustion process, reached levels that pose potential respiratory risks, challenging the perception of scented wax melts as a benign household product,” Jung says.

Read More: Air Pollution Is Damaging Your Brain Faster Than You Think

Do All Scented Products Cause Indoor Pollution?

While the team’s recent study looked at flame-free scented candles, previous work from Jung examined the impact that other fragrant products have on indoor air quality.

Her team found in an earlier study that hair products like sprays persisted for a while indoors, especially after being exposed to devices like hair curlers or straighteners.

In fact, Jung’s work shows that scented products in general are significant contributors to indoor pollution. In one study they found that scented products can create more breathable nanoparticles than gas stoves or diesel engines.

It may not be limited to homes. Scented products like air fresheners often used in cars release many of the same volatile organic compounds to mask lingering stench — all in a relatively smaller area than your average home. But Jung hasn’t specifically studied these potential impacts, and said further research would be needed to get a clearer idea of any problems they might be causing.

Health Problems from Scented Products

The types of health problems these chemicals can cause isn’t entirely clear, but they may pose issues for our breathing systems — some of them long-term.

“Some [volatile organic compounds] are classified as hazardous air pollutants, while airborne nanoparticles have been linked to lung inflammation, cardiovascular effects, and other adverse health outcomes,” Jung says.

She noted that actual exposure from these potentially harmful chemicals might vary based on a number of factors, though.

Read More: 5 Eco-Friendly Decorations for the Holiday Season

What to Avoid

Other recent research has found homes vary greatly in their amount of indoor air pollution. Ventilation, occupancy patterns, and household location can all effect how polluted homes are. The authors of that paper say that monitoring indoor pollution in your home is becoming increasingly important, as a result.

“With more time spent working from home, understanding the factors that affect air quality within households is increasingly important,” said Owain Rose, a coauthor of the paper, in a press release.

Jung has recommended always keeping on exhaust fans such as those above stoves or in bathrooms when using these products. But the best thing would be to avoid hair care products, or scented candles, and waxes altogether.

Article Sources

Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:

Joshua Rapp Learn is an award-winning D.C.-based science writer. An expat Albertan, he contributes to a number of science publications like National Geographic, The New York Times, The Guardian, New Scientist, Hakai, and others.