Researchers said the new understanding of the female body's response to sex shed light on proactive aspects of the immune system beyond simply attacking invaders which threaten health. Photo by Sebastian Kaulitzki/Shutterstock

BLOOMINGTON, Ind., Oct. 7 (UPI) -- Although couples trying to have a baby are generally told to have sex as much as possible whether or not a woman is at a point in her menstrual cycle to be fertile, until now, how this helps increase the chance for pregnancy was unknown.

Sexual activity triggers responses from the immune system in a woman's body to prepare it for the possibility of pregnancy, according to two new studies. The studies not only shed light on the mechanics of pregnancy, but researchers said they may also impact treatment of autoimmune disorders.


"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," said Tierney Lorenz, a visiting research scientist at the Kinsey Institute at Indiana University, in a press release. "This research is the first to show that the sexual activity may cause the body to promote types of immunity that support conception."

In the studies, which are published in the journals Fertility and Sterility and Physiology and Behavior, researchers recruited 30 women for the studies, half of whom were sexually active and half abstinent. The studies report on differences in levels of helper T cells and antibodies in the two groups of women, of which there are multiple types of each.

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Helper T cells activate cells in the body to fight invading microbes, while antibodies called immunoglobin are secreted by white blood cells to fight foreign invaders. In the case of pregnancy, type 2 T cells help the body not reject sperm or an emerging embryo that could be misconstrued as a threat to health. Immunoglobin G anitbodies can fight disease without interfering with the uterus, unlike immunoglobin A antibodies found in the mucous in the reproductive tract that interfere with fertilization.

Researchers found in the studies that sexually active women had higher levels of both types of pregnancy-agnostic immune cells during the luteal phase of the menstrual cycle, when the uterine lining thickens in for pregnancy. Women in the study who were not sexually active did not show the immune changes.

"We're actually seeing the immune system responding to a social behavior: sexual activity," Lorenz said. "The sexually active women's immune systems were preparing in advance to the mere possibility of pregnancy."

Lorenz said the research sheds light on the immune system overall, based on the proactive response to prepare for the entrance of a foreign body, as opposed to activating to fight off a threat. Additionally, understanding that sex can cause natural changes and effect blood test results may help doctors treating immune disorders.