Mental stress that some students experience while taking their exams may also affect their immune defence system, more so if they have an allergy , scientists say.

Mats Lekander and other scientists at Karolinska Institutet in Stockholm examined how a major medical exam affects stress hormone levels, the immune system and lung function among students with and without allergies .

The extensive tests were conducted on two occasions - first during a calm period with no exam in sight, and then shortly before a major exam, according to a report that appeared on the institute's website.

Twenty-two students with hay fever and/or asthma and 19 healthy students took the test.

The researchers found that there are important links between mental stress and the complex physical inflammation reactions characteristic of allergies.

For the first time on record, scientists were able to show that a group of cells that are central to the human immune system - known as regulatory T cells - appear to increase sharply when the students were under stress.

A regulatory T cell is a kind of white blood cell that controls the activity of a number of other types of immune cells . This increase was observed in both groups of students.

The study also showed that blood concentrations of a group of inflammation products called cytokines had changed and shifted against a pattern associated with allergic inflammation in students with allergies, but remained normal in healthy students.