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This column appears in the October 25, 2015 issue of The New York Times Magazine

For decades, doctors have recom­mended that couples trying to conceive should have intercourse as often as possible, not only during ovulation (obviously) but at other times as well. Doing so is known to improve the odds of success, though the reasons have remained a puzzle. Now two new studies suggest, somewhat radically, that sex alters a woman’s immune system in ways that affect her chances of conceiving.

The data for the studies come from the Kinsey Institute for Research on Sex, Gender and Reproduction and the Center for the Integrative Study of Animal Behavior, both at Indiana University, where researchers recruited 30 healthy, premenopausal women, all of whom were heterosexual and not trying to become pregnant. About half of them were sexually active, using condoms or intrauterine devices as birth control, while the others were abstinent. The volunteers provided blood, saliva and other samples throughout several menstrual cycles. Apart from their sex lives, women in both groups were broadly similar in terms of health and lifestyle.

They soon revealed distinctly different immune-system responses over the course of a menstrual cycle. According to one of the new studies, published in September in the journal Fertility and Sterility, the sexually active women displayed heightened levels of a certain immune cell when their reproductive systems were preparing to release an egg but before they were able to become pregnant. Later in the menstrual cycle, when conception was possible, these women developed higher levels of a different type of immune cell — one known to help a body recognize and ignore nonhazardous foreign cells, like those in a fetus. There were no similar changes in the immune systems of the abstinent women.

The other study, published in Physiology and Behavior, found other immunological differences. Sexually active women early in their cycles developed more antibodies of a type that lives in the mucus lining the reproductive tract and represents a threat to sperm and fetuses. Levels of these antibodies dropped later in the cycle, while the numbers of a different germ-fighting antibody in the blood but not in the reproductive tract grew.

Together, these findings indicate that ‘‘the more frequently a woman engages in sexual activity, the more often her immune system gets the message that it’s time to reproduce,’’ says Tierney Lorenz, the research scientist at the Kinsey Institute who was the lead author of both studies. The research did not examine conception or rates of illness and infection, so the real-life implications of the differences in immunity remain uncertain. It’s also not clear how the immune system knows someone is having sex. There may be messages from the brain or extra changes in hormones, Lorenz says. Sexual partners may even exchange elements of their microbiome, prompting changes in each other’s immunity. For now, the primary lesson Lorenz takes away from these studies, she says, is ‘‘awe for the elegance of our flexible, powerful, socially aware immune system.’’

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