At last some good news: sex is good for you, at least in moderation. Psychologists in Pennsylvania have shown that the immune systems of people who have sex once or twice a week are boosted. Scientists 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," says Carl Charnetski of Wilkes University. IgA binds to pathogens at all the points of entry to the body, then calls on the immune system to destroy them.
To find out if sex can alter IgA levels, Charnetski and his colleague Frank Brennan asked 111 Wilkes undergraduates, aged 16 to 23, how frequently they'd had sex over the previous month. They also measured levels of IgA in the volunteers' saliva.
The results showed that participants who had sex less than once a week had a small increase in IgA over those who abstained completely. Those who had one or two sexual encounters each week had a 30 per cent rise in levels of the antigen. But, perhaps unfortunately, 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, thinks the high levels of IgA in volunteers who had moderately frequent sex are easy to understand. "Sexually active people may be exposed to many more infectious agents than sexually non-active people," Lowell says. "The immune system would respond to these foreign antigens by producing and releasing more IgA." However, why there was no IgA rise in the most sexually active group is less clear. "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," speculates Charnetski. "We know that stress and anxiety make IgA go down."
