Early Warning System: How Our Immune Systems Respond Even Before an Infection

Our fight-or-flight response tells us what to do if we spot an imminent threat. But what about the dangers that we can’t see? Thanks to a new report from scientists in Switzerland, we now know that the same protective response works to keep us safe from infectious disease pathogens.

We may not be able to see the pathogens themselves, but we can often tell from coughing, rashes, and other symptoms that people are sick. In this study, scientists wondered whether the very sight of a sick person could stimulate a protective immune response. (Short answer: yes!)

This must have been an entertaining study to participate in. Scientists evaluated nearly 250 people, outfitting most of them with virtual reality headsets. (There was a small control group who had received a flu vaccination; these people were included only to test their immune response to the vaccine, so they didn’t go through the VR process.)

Through the VR, participants watched as avatars of healthy, fearful, or visibly sick people approached them. As these avatars appeared to get closer and closer, imaging tools like EEGs and fMRI were used to measure brain activity. Even from greater perceived distances from the sick avatars, participants’ brains lit up: increased activity in the fronto–parietal areas triggered a cascade of biological reactions. This response was much more noticeable when a participant encountered the sick avatar than the fearful avatar, and from farther away.

Scientists also collected blood samples shortly after the VR session to analyze markers in the immune system. Unlike participants who saw the healthy or fearful avatars, those who saw the sick avatar had a measurable immune response. The cell types that are essentially the immune system’s first responders fired up, displaying patterns similar to what’s seen when a person has actually encountered an infectious disease pathogen. The comparison of recently vaccinated participants to participants viewing the sick avatar found similar levels of immune markers; both groups’ markers were elevated compared to participants in control groups.

This might sound like a bit of a lark, but the study was very well controlled. For example: all participants were right-handed, with no reported history of relevant immune or neurological disorders, and all testing was performed at the same time of day. Immune markers measured in blood tests after the VR experiments were compared to those in blood samples collected prior to any encounter with the sick-person avatar. And, of course, groups of participants were compared — some never saw the sick avatar at all.

The scientists report that “the behavioral immune system evolved to minimize false-negative responses and is exquisitely sensitive to cues that superficially resemble the symptoms and signs of pathogen infections.” This response enables our bodies to trigger the start of an immune response even before an infection occurs, providing valuable time to protect us from danger.