A new study has found that the key to solve female infertility may be lots of sex.

Scientists believe that sex alters the immune system in the body in a way that can solve an issue of female fertility — good news for those who have been struggling to conceive, according to researcher from the Center for Integrative Study of Animal Behavior at Indiana University – Bloomington.

Researchers say that sexual activity results in physiological changes in the body that increase the changes a woman has of getting present even when it’s not her ovulating period.

Tierney Lorenz, a visiting research scientist at the Kinsey Institute, is the lead author on the study, and was assisted by Julia R. Heiman, a professor in the Department of Psychological and Brain Sciences, and Gregory E. Demas, a professor in the Department of Biology. All of the researchers work at the Center for Integrative Study of Animal Behavior at Indiana University – Bloomington, with Heiman additionally affiliated with the Kinsey Institute, where she was once the director. The scientists base the results on data produced by participants in the Kinsey Institute’s Women, Immunity, and Sexual Health (WISH) study, which aimed to collect a significant amount of data from 30 healthy women who were in their menstrual cycle, with half being sexually active and the other half being abstinent.

“It’s a common recommendation that partners trying to have a baby should engage in regular intercourse to increase the woman’s changes of getting pregnant — even during so-called ‘non-fertile’ periods — although it’s unclear how this works,” Lorenz said. “This research is the first to show that the sexual activity may cause the body to promote types of immunity that support conception. It’s a new answer to an old riddle: How does sex that doesn’t happen during the fertile window still improve fertility?”

Previous studies have shown that changes in immune function during pregnancy and after childbirth, as well as during parts of the menstrual cycle, but this is the first time scientists have shown that sexual activity can also play a role — women who have significantly more sex are also significantly more likely to have very distinct differences in their immune system regulation.

The studies may also solve the mystery of why some women experience fluctuations in their immune response while others do not, and the revelation that sexual activity could be behind could be a huge help to physicians dealing with patients who have immune disorders. Sexually active women appear to show greater changes in their helper T cells — which manage the immune response in a woman’s body by alerting cells of invading microbes and getting them to attack and destroy them — as well as differences in antibody levels known as immunoglobulins, which are produced by white blood cells and help prevent harmful diseases from establishing a foothold in the body.

“The female body needs to navigate a tricky dilemma,” Lorenz said. “In order to protect itself, the body needs to defend against foreign invaders. But if it applies that logic to sperm or a fetus, then pregnancy can’t occur. The shifts in immunity that women experience may be a response to this problem.

“We’re actually seeing the immune system responding to a social behavior: sexual activity,” she added. “The sexually active women’s immune systems were preparing in advance to the mere possibility of pregnancy.”

Type 1 helper T cells may be key to understanding this, as they help the body defend against threats from the outside, while Type 2 helper T cells protect things that are helpful to a pregnancy that might otherwise be classified as an invader by the immunoglobulins, which would otherwise be focused on keeping out things like sperm or even a growing embryo. These antibodies are found in the mucus of the reproductive tract of a woman, and therefore are a big risk for blocking sperm and other things that may be necessary for fertilization. Other types of immunoglobulin are found in the bloodstream and attack diseases without interfering with the reproductive process.

In women who were sexually active, there’s good news: there were a lot more of these type 2 helper T cells protecting things key to fertilization than in women who did not have sex very often or at all, especially during the luteal phase of the menstrual cycle, which is when the uterine lining thickens in preparation for pregnancy. The type 1 helper T cells were found in greater abundance in women during the follicular phase of the menstrual cycle, which is when the follicles of the ovaries are maturing.

Sexually active women appear to show big changes in immunoglobulins, particularly in immunoglobulin G — the antibodies present in the bloodstream — during the critical luteal phase, and immunoglobulin A when the ovaries were in the follicular phase, whereas this change was not observed in women who weren’t sexually active.

It is all part of mounting evidence that the immun system actually adapts to external cues rather than acting as a passive system that can be affected by anything. As it turns out, changes in both social and environmental behavior can have a big impact in how our immune system behaves, and could lead to a breakthrough in treatments for autoimmune disorders in patients, as well as help those who are trying to get pregnant become more successful at conceiving.

The findings were published in the journal Fertility and Sterility and the journal Physiology and Behavior.