In a recent study, McGill University researchers studied mice that lacked an enzyme called Caspase-12. This gene is known to stop the inflammatory process, which is the body's natural mechanism and first line of defense against invading bacteria and viruses. Without the Caspase-12 gene, the mice were unable to resist infection. When the researchers implanted the human Caspase-12 gene into both male and female mice, only the males became more prone to infection. As a result, they surmised that oestrogen in women seems to block the production of Caspase-12. Because the experiments were conducted using a human gene, the researchers believe their work will be applicable to humans.

Says lead researcher Dr. Maya Saleh, "We were very surprised by these results, and we determined that the estrogen produced by the female mice blocked the expression of the human Caspase-12 gene. We were also able to locate where the estrogen receptor binds on the gene in order to block its expression, which indicates that the hormone exerts direct action in this case. These results demonstrate that women have a more powerful inflammatory response than men."

So why do scientists believe women have developed more robust immune systems? The immune systems of women may have evolved to better protect women's reproductive role. "In evolutionary terms it only takes one male to reproduce with lots of females, but females are much more important in terms of producing offspring," says Dr. Leslie Knapp, of the University of Cambridge, who points out that there is a substantial body of evidence demonstrating that women are better at fighting off infections than men. "Women are well known to be able to respond more robustly to infections, and to recover more quickly than men," she says.

One important question still needs to be asked, say the researchers: If this work opens the doors to using genetic manipulation to reinforce the immune system, "will men be amenable to the idea of being treated with an exclusively female hormone?" That remains to be seen. The results have been published in the Proceedings of the National Academy of Sciences.

News Release: Women fight off disease better www.news.bbc.co.uk May 13, 2009