Men are more prone to catching colds and flu because of changes laid down during puberty, according to research out yesterday.
The differences in the male and female immune response means that men have a lower "immediate defence" against viruses.
Women, on the other hand, are more prone to autoimmune diseases like multiple sclerosis and rheumatoid arthritis.
Researchers in the United States found that these differences were laid down during puberty and are controlled by the female sex hormone oestrogen.
They examined 12,000 genes in the spleen of male and female mice before, during and after puberty.
Males and females had similar immune systems when young but changes come about during puberty, they found.
At that stage, males had weaker adaptive immunity - the system which wards infections caused by bacteria or viruses.
A higher level of adaptive immunity in females meant they were better at fighting those infections.