Men have a reputation for being sooks when it comes to winter colds and flu but researchers believe "man flu" really does exist and the culprit is testosterone.
US and French scientists have found evidence that testosterone plays a key role in making men's immune responses more sluggish than women's.
It has long been thought that women's immune systems are generally stronger and respond more quickly to infections and vaccinations than that of males, but the cause has been unknown.
The new study blames testosterone for dampening the immune response, suggesting men with high testosterone are more likely to feel the pain of the flu and for longer.
Researchers from Stanford University reported in the medical journal Proceedings of the National Academy of Sciences a difference in how men and women responded to flu vaccines, which are given to trigger an immune response so the body produces antibodies to certain viruses. The researchers measured the immune response in 53 women and 34 men after their annual flu shot, finding that women produced a higher antibody response and bigger amounts of substances known as inflammatory cytokines.
When the researchers tested the volunteers' blood, they found a set of genes that controlled immune responses and appeared to be regulated by testosterone. Men with high testosterone levels had the weakest antibody response.