It has to do with the immune system.
Tierney Lorenz, a visiting research scientist at the Kinsey Institute at Indiana University, theorizes that sex appears to trigger physiological changes that may be preparing a woman for pregnancy.
In the first study, which involved 30 women, Lorenz and her colleagues found sexually active women experienced greater changes in what are known as "helper T cells," as well as the proteins that T cells use to communicate. In the second, which involved 32 women, the researchers found similar differences in how antibody levels fluctuate between the two groups.
Helper T cells are a key part of the body's immune response, activating cells that defend the body against microbes. Antibodies, or immunoglobulins, are secreted by white blood cells and fight infection from bacteria, viruses and other attackers.
"We're actually seeing the immune system responding to a social behavior: sexual activity," Lorenz said in a statement. "The sexually active women's immune systems were preparing in advance to the mere possibility of pregnancy."
The researchers wrote that their findings appear to show that women's physiology "navigates tradeoffs between reproduction and immunity."
The studies, which involved taking saliva samples and blood samples from the women -- about half of whom were sexually active and half of whom were not -- found that levels of helper T cells fluctuated throughout a women's menstrual cycle. During the luteal phase, which is when the uterine lining thickens in preparation for pregnancy, they found higher levels of type 2 helper T cells in sexually active women than in those who had abstained from sex. During the follicular phase, during which the ovaries' follicles are maturing, researchers found higher levels of type 1 helper T cells.
They found a parallel change in antibodies. During the luteal phase there were higher levels of immunoglobulin G, and during the follicular phase, higher levels of immunoglobulin A.
"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."
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