Psychologists in Pennsylvania have shown that people who have sex once or twice a week get a boost to their immune systems.

Scientists can evaluate how robust our immune systems are by measuring levels of immunoglobulin A (IgA), an antigen found in saliva and mucosal linings.

IgA is the first line of defence against colds and flu. It binds to bacteria that invade the body, and then activates the immune system to destroy them.

Dr Carl Charnetski, of Wilkes University in Wilkes-Barre, and his colleague Frank Brennan investigated the effect that sex had on IgA levels.

They asked 111 Wilkes undergraduates, aged 16 to 23, how frequently they had had sex over the previous month. They also measured levels of IgA in the volunteers' saliva.

According to the New Scientist, the results showed that participants who had sex less than once a week had a tiny increase in IgA over those who abstained completely.

Those who had one or two sexual encounters each week had a 30% rise in levels.

But people who had very frequent sex - three times a week or more - had lower IgA levels than the abstainers.

Clifford Lowell, an immunologist at the University of California at San Francisco, said: "Sexually active people may be exposed to many more infectious agents than sexually non-active people.

"The immune system would respond to these foreign antigens by producing and releasing more IgA."

The reasons why more sexually active people did not experience a rise in IgA are less clear.

Dr Charnetski said: "My feeling is that the people in the very-frequent-sex group may be in obsessive or poor relationships that are causing them a lot of anxiety.

"We know that stress and anxiety make IgA go down."

Dr Douglas Fleming, who heads the Royal College of General Practitioners' influenza research unit in Birmingham, expressed doubts about the study.

"Have they considered the question that healthy people are more likely to have sex regularly anyway?" he asked.

Professor Ron Eccles, director of the Common Cold Centre in Cardiff, said it was not good science to equate IgA levels with the body's ability to fight off cold infections.

He said: "There are no general measures that can quantify resistence to infection.

"It is not impossible that sex has an effect on the body's ability to fight off cold, but it would be very difficult to substantiate that claim from this data."