Women not only live longer than men, they also appear to be in more robust health. A new hypothesis offers a reason why: it's in their genes.

Women are known to have a lower incidence of cancer — men have a two- to five-fold greater risk of developing the disease. Women are also better able to survive trauma, and, according to some reports, don't get as seriously ill from bacterial and viral infections.

In a new paper, researchers from Ghent University in Belgium argue these sex-specific health disparities may be due, at least in part, to tiny pieces of genetic material called microRNAs. The main function of microRNAs in cells is to turn off, or "silence," specific genes. The researchers say microRNAs located on the female X chromosome may give women an immune system advantage over males.

While the researchers' idea is certainly debatable, the paper "raises awareness of how little we consider the influence of sex on immune responses," said Eleanor Fish, a professor of immunology at the University of Toronto in Canada, who was not involved in the work.

Often, researchers who conduct medical studies on people do not analyze their data by sex, and sometimes they don't report the sex of patients at all. Hopefully this will change, Fish said, so that every time a study is done, sex differences are considered, she said.

XX and XY

In humans, sex is genetic: Females have two X chromosomes, while males have one X and one Y. However, in females, one X chromosome in each cell in the body is randomly shut off, or inactivated, while the embryo is developing.

But X inactivation is not a perfect process, and sometimes genes on the X chromosome escape inactivation. In this case, a female ends up with two active copies of a particular gene.

Here is where the researchers think the microRNAs come in. The X chromosome contains 10 percent of all microRNAs in the human genome. The Y chromosome has none. Some of the microRNAs on the X chromosome are thought to be involved in immune system function and cancer development.

If a microRNA did something "good," like help control cell growth, having two copies of that microRNA might provide females with extra protection against cancer. The same would be true for microRNAs that played a role in immune function.

As a real-life example, septic patients (who have widespread bacterial infections) have low levels of a particular microRNA found on the X chromosome, the researchers said.Thus, this particular microRNA may offer some protection against sepsis.

The researchers said they need to do more work to support their theory. For example, it's not known whether microRNAs on the X chromosome "escape" inactivation, they said.

Other factors

The X chromosome is known to contain a number of genes related to health, Fish said, and adding microRNAs to the mix would suggest that the X chromosome is even more important in terms of health differences between men and women.

However, the X chromosome is far from the only reason for the strong immune response in females, Fish said. Hormonal differences and a number of other factors probably play a role, she said.

Researchers have only recently turned their attention to mircoRNAs, and there is likely an incredible amount of information we can learn from them, Fish said.

The paper is published in the journal BioEssays.

Pass it on: Women's seemingly superior immune systems might come from having more microRNAs in their cells than men.

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