A new study has found that moving in with your partner isn’t just life-changing, but body-changing.

Most of your body’s internal systems are similar to those of other people. But not your immune system. Environmental factors—your diet, your lifestyle, and, of course, all the infections you’ve ever had—alter the types and numbers of immune cells residing in your body, making your immune system a unique record of the life you’ve lived. In adults, at least, only about 25% of that variation is determined by genes.

Yet we don’t know much about how those environmental factors change the immune system. This matters because medical researchers are increasingly realizing that tiny differences in the immune system can make us more or less susceptible to diseases such as diabetes or dementia.

For the study, published in Nature Immunology, a team led by Adrian Liston at the University of Leuven examined the immune systems of 700 people by taking blood samples. Then they tracked around 150 of those people over six months, seeing how their immune systems changed in response to changes in the environment. There were two surprising findings.

The first was that there doesn’t seem to be much difference between the immune systems of men and women of the same age. This is surprising because past studies have, for instance, shown that women suffer from auto-immune diseases more than men do. ”It may be that the tiny differences that do exist between the sexes have a large impact on women as they age,” Liston told Quartz.

The most unexpected discovery, however, was that people in couples are remarkably similar. The 70 married couples in the sample showed, on average, 50% less variation in their immune systems than randomly paired men and women of a similar demographic.

“Though we didn’t study unmarried couples, I believe cohabiting couples would show similar results,” Liston said.

Living with someone leads to a litany of small changes: diet, alcohol intake, and exercise routines converge. So do exposure to pollution and infections. Even the couple’s microbiomes—the millions of microbes living in and on their bodies—become more similar. A 10-second kiss, for instance, is a conduit for exchanging 80 million bacteria of about 300 species.

All these things, Liston reckons, contribute to making a couple’s immune systems more similar.

Ultimately, knowing more about the immune system can help us tweak it in just the right away to treat and even prevent diseases. This is why immunotherapy is one of the hottest areas in medical research.