Women have long complained that men exaggerate a case of the sniffles, with even the most macho of males insisting he doesn't just have a cold and it's worse than it appears. But recent research may lend some truth to the tale of the "man flu," claiming a weaker immune system in some men makes them more vulnerable to the virus. 

A team of researchers led by Mark Davis -- a professor of microbiology and immunology at the Stanford University School of Medicine and the study's senior author -- examined gender-related differences to the flu vaccine. The team measured immune responses in 53 women and 34 men after they received a seasonal flu vaccination, and found the women produced a higher antibody response than the men.

The study, published Monday in the Proceedings of the National Academy of Sciences, also identified a set of genes that are regulated by testosterone and may be the cause of men's weaker immune systems. The researchers found a link between testosterone levels and the men's immune response: Those with higher levels of the male hormone had weaker -- or even nonexistent -- immune responses to the vaccine.

"This is the first study to show an explicit correlation between testosterone levels, gene expression and immune responsiveness in humans," Davis said in a statement. "It could be food for thought to all the testosterone-supplement takers out there."

It's been known that men tend to be more susceptible to viral and and bacterial infections, although the reasons why this happens are still unclear. Women have had more robust responses to vaccines for other diseases as well -- including yellow fever, measles, mumps, rubella and hepatitis -- but the Stanford study was the first to find a connection between testosterone levels and a weakened immune system.

The researchers found that women on average had higher levels of proteins that trigger inflammation, which is a common immune response. But testosterone is known to have anti-inflammatory properties, the study says. 

Although neither the pro-inflammatory proteins nor the testosterone levels appear to have a direct connection to a person's response to the flu vaccine, the researchers found testosterone controls the expression of the genes -- known as Module 52 genes -- that weaken men's immune responses.

Still unclear, however, is why a hormone would enhance some male characteristics, yet also weaken the immune system. It may be because men are more likely to become injured from activities that could increase their risk of infection. While some immune response is necessary, too much of a reaction can be more dangerous than the infection alone, according to the study.

"Because males of many species are more likely to experience trauma than females, this positive effect of testosterone may also help to balance out the consequences of reduced immunity to infection," the study says.