Immune cells are more paranoid than we thought: reacting to any sign of illness

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EXCERPTS: The best immune systems thrive on a healthy dose of paranoia. The instant that defensive cells spot something unfamiliar in their midst—be it a living microbe or a harmless mote of schmutz—they will whip themselves into a frenzy, detonating microscopic bombs, sparking bouts of inflammation, even engaging in some casual cannibalism until they are certain that the threat has passed. This system is built on alarmism, but it very often pays off: Most of our encounters with pathogens end before we ever notice them.

The agents of immunity are so risk-averse that even the dread of facing off with a pathogen can sometimes prompt them to gird their little loins. Ashley Love, a biologist at the University of Connecticut, has seen this happen in birds. A few years ago, she stationed healthy canaries within eyeshot of sick ones, infected with a bacterium that left the birds sluggish and visibly unwell. The healthy canaries weren’t close enough to catch the infection themselves. But the mere sight of their symptomatic peers revved up their immune systems all the same, Love and her colleagues report today in Biology Letters.

[...] The connective tissue that links visual cues to immune activation is still scientifically foggy. At first, “it all seems kind of magical,” Schaller, the University of British Columbia psychologist, told me. But it’s also sensible (literally) for animals to glean information from their environment and react accordingly.

[...] The paper speaks to the strange appeal of visible disease, says Cécile Sarabian, an expert in sickness behaviors at the Kyoto University Primate Research Institute who wasn’t involved in the study. The signs and symptoms of infection are often a pain for the individual who experiences them. But they also “alert others, and prepare other potential hosts,” she told me.

Spotting symptoms alone isn’t good enough. In the past year and a half, SARS-CoV-2 has benefited from its ability to spread silently from person to person. Humans have also taken a multitude of other measures—masking, distancing, and the like—to keep the coronavirus at bay, acts of avoidance that Schaller says count as a kind of behavioral immunity. Still, Schaller and others think it’s interesting to consider what sorts of infections count as truly “asymptomatic.” Even if an infected person isn’t feeling outright ill, they might be beaming out slight signals that betray their status, and influencing those around them. “We’re pretty sensitive to some pretty subtle stuff,” Schaller said. “It could be that we are able to pick up on other people’s sicknesses, even if those people are not yet aware.” (MORE - missing details)

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