Women may have stronger immune systems than men thanks to their second X chromosome, scientists suggest.

Since men have only one X chromosome, their immune systems may be at a disadvantage, scientists say. (iStock) Writing in Wednesday's issue of the journal BioEssays, Claude Libert, from Ghent University in Belgium, and his colleagues hypothesized that the reason women seem better able to fend off attacks on the immune system like from the flu could be because of the two X chromosomes females have.

MicroRNAS are tiny strands of RNA. Some have important roles in immunity and cancer, Libert said.

The researchers believe that differences in how microRNAs interact with genes may partly explain why women live longer than men on average, and appear to fight sepsis, infection and trauma better.

Their hypothesis is that the microRNAS block immunity genes on a man's X chromosome. Since men have only one X chromosome, their immune systems may be at a disadvantage compared with women who have two. If one of a woman's immunity genes are silenced, she has a backup source.

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The researchers produced a detailed map of all microRNAs (miRNAs) that are known to have a role in immune functions and cancer in human and mouse X chromosomes to support their idea.

"We believe this immunological advantage is due to the silencing of X-linked genes by these microRNAs," Libert said in a release.

The study's authors described several open questions that remain.

"The myriad of functions and targets of miRNAs has obvious implications in the development of effective therapeutic strategies for cancer and immunity, and is likely to represent a challenge for researchers of both fields in the coming years," they concluded.