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Our veins are swimming with immune cells of many different kinds. Some bear the memory of previous infections, in case we should encounter them again; some are actively fighting invaders; others are merely on the look-out. Counting all of the varieties of cells and what molecules they are producing gives researchers a profile of someone’s immune system. In a study of more than 600 people, published recently in Nature Immunology, just such an analysis revealed that people’s immune systems are incredibly stable over time—even getting a vaccine or a stomach upset changed things only briefly, before the immune cells returned to normal. It also showed that almost the only thing that will change a given person’s normal seems to be living with a partner.

Maintaining a relationship isn’t the best way to maintain cleanliness.Photograph by Everett Collection | Shutterstock
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The researchers began by recruiting 638 people, including 70 pairs of parents currently raising children. Over the course of three years, they took nearly 1,000 samples from this group—mostly from a cluster of 177 people who were sampled on average every six months. Some traveled to places where they picked up traveler’s diarrhea for a few days, and the researchers made sure to get samples after they returned. Others received vaccinations for various diseases, which provoke the immune system to produce cells that will come in handy should the person encounter the microbes that cause those maladies. Despite these perturbations, the team found that people’s own immune profiles showed little permanent change.

Looking to see how much variation there was between individuals, however, the researchers compared pairs of people to each other, and here they found something interesting: Each co-habitating parent in the group was clearly more similar to their mate than they were to any other person. The team didn’t take samples before the couple began living together or before their children were born, so the researchers didn’t actually see the process of adaptation take place. But each pair of parents was so much more similar than any two random people—with 50 percent less variation between them than strangers—that the researchers believe something about their shared lives caused this mutual accommodation.

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Could these people’s immune systems be converging because their microbiomes are adapting to their shared environment? The immune system must maintain a relationship with friendly microbes to keep them straight from the bad ones, so if partners are exposed to similar bacteria and viruses, that exposure could make their populations of immune cells more similar. The researchers point out that it’s already known that couples who live together have more similar microbiomes than strangers do, perhaps because they swap bacteria with each other or share lifestyle choices like smoking or drinking. “Some of these factors are likely to be even more shared after children,” says Adrian Liston, the senior author and a professor at University of Leuven in Belgium. “For example, children are likely to increase the exchange of gut bacteria by reducing the sterility of the household (to put it nicely).”

Not that human houses are necessarily paragons of sterility before kids: As microbiome researcher Jack Gilbert told a Science reporter recently, “Your trousers or your pants are like a loose fish net material to bacteria. As soon as you sit down, your bottom or your vaginal microbiota is expelled onto that surface and it is actually reasonably persistent until the next person sits down.” We are likely sharing liberally with everyone we live with.

Your lover, or even your roommate, might share more with you than you might have expected.

Since the team did not compare the parents to couples who live together but do not have kids, only to random strangers, they can’t really say how much of what they’re seeing is the effect of the offspring and how much the effect of sharing a space. The same thing could be happening to couples without kids—or just people living in close proximity. “In the end, the effect is certainly going to depend on couple to couple. I expect some couples to converge very rapidly even before having children; other couples may slowly converge and then rapidly change after children; and so forth,” says Liston. “I would even expect to see some convergence between individuals who live together without being in a relationship.”

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In this view of reality that’s coming into focus, your lover, or even your roommate, might share more with you than you might have expected. Your immune systems may be adapting to your shared environment, drawing you closer than strangers ever are.

The new finding recalls an older one that continues to be a cocktail-party conversation starter 20 years after its publication: the sweaty T-shirt study. You might recall this experiment, in which scientists asked women to smell T-shirts that men had slept in for two nights and to rate them according to “sexiness, pleasantness, and intensity.” The study found that the shirts women rated the most pleasant were from men with major histocompatibility complexes (MHCs)—a collection of proteins used by the immune system to tag invading pathogens for destruction—quite different from the women’s own. The findings seemed to indicate that women might seek out mates who have different immune systems from themselves, perhaps so that a couple’s children would inherit a variety of proteins—and that somehow the scent of sweat included information about the men’s MHCs.

The current study is looking at the immune system in a totally different way—measuring the overall number and kind of cells, not the nature of specific proteins. And the 1995 study is perhaps best considered a source of questions rather than answers: It was small, and a few studies since then have not left things much clearer.

But together the studies leave an interesting suggestion: We might seek each other out for our differences, only to grow more similar the closer we get.

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Veronique Greenwood is a science writer and essayist. Her work has appeared in The New York Times Magazine, Discover, Aeon, New Scientist, and many more. Follow her on Twitter @vero_greenwood.

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