Researchers have discovered that physical and emotional enjoyment, even in small doses, can enhance immune function for hours afterwards.

They believe that life's small pleasures may have a cumulative effect in boosting the immune system over a long period.

And how did they test their theories? By subjecting people to the smell of chocolate.

The research was carried out by ARISE (Associates for Research Into the Science of Enjoyment) an international group of scientists and academics from the fields of physiology, psychology, psychopharmacology and sociology.

Two separate studies measured how much of an antibody, secretory Immunoglobulin-A (sigA), was produced when people had pleasant and unpleasant experiences.

SigA, found in the saliva, protects against respiratory infections.

The first study examined how the immune system responds to happy and guilty memories. Happy thoughts showed a marked improvement in mood and clearly increased immune response, while guilty ones were shown to undermine it.

The amount of SigA secreted doubled 20 minutes after happy thoughts and grew further after 45 minutes. Even three hours later the immunity level was 60% higher than when the experiment started.

The second study showed that unpleasant odours, such as rotting meat, reduce the amount of SigA that is produced - potentially weakening resistance to disease.

Conversely, pleasant smells, such as chocolate, may stimulate SigA production, enhanching immune protection.

The research also found significant differences in the way men and women react to odours.

Women were more likely to be disturbed or soothed by the different odours, but the actual impact on their immune system was weaker than in men.

Listen to your body

Rotting meat: a health hazard

Professor David Warburton, found of ARISE and head of psychopharmacology at Reading University, said: "Previous scientific experiments have observed a correlation between changing moods and the immunity system, but these new studies provide a direct causal link. Identifying this direct link proves that happiness could make you healthier.

"Instead of worrying about the often ill-founded health scares created by so-called health experts most people would do better to listen to their bodies.

"These studies illustrate how our bodies naturally seek to protect themselves from disease by doing the things we enjoy."

Professor Warburton said two other new studies had showed that the body has some self-regulating systems to ensure that people do not take pleasures to excess.

In one study people were asked to eat "special" food such as chocolate on a regular basis. It was found that their appetite for the special foods diminished with regular exposure, while their appetite for staples such as bread and butter remained constant.

The final study found that brain receptors activated by sweet tastes become less sensitive with repeated exposure.

Professor Warburton said: "Rather than worrying about whether or not you should be indulging in the things you enjoy doing, it is probably healthier just to get on with them, albeit within moderation."