Getting stressed now and again may be good for your health, research suggests.
A little stress may actually be good for you
A short burst of stress, such as that caused by giving a speech, may strengthen your body's immune system.
But long-term stress, such as living with a permanent disability, may render you less able to fight infections, say the study authors.
Dr Suzanne Segerstrom and Dr Gregory Miller report their findings in the journal Psychological Bulletin.
Scientists have known for some time that stress can have a negative effect on the body.
Now the American and Canadian pair from the University of Kentucky and the University of British Columbia say some psychological stress can be good for you.
They looked at about 300 scientific papers published on the subject, involving almost 19,000 people.
Stressful situations that lasted only short periods appeared to tap into the primeval 'fight or flight' response, which dates back to when early man was threatened by predators.
This response benefited the person by boosting their body's natural front-line defence against infections from traumas such as bites and scrapes.
But long-term anxiety had the opposite effect.
Situations that caused permanent stress and turned the person's world upside down were damaging to health.
These stressful events, such as caring for someone with dementia, appeared to wear out the immune system, leaving the person prone to infection.
Other damaging experiences included losing a partner or spouse or being abused as a child.
The important factor appeared to be knowing that the event causing the anxiety would end soon.
Some people seemed to be more vulnerable to stress than others.
Older people and people who were already ill were more likely to suffer damage to their immune systems.
Co-author Dr Miller said: "Older people are definitely more vulnerable to stress.
"Either the mind or the body seems to lose some of its ability to fight back.
"The issue now is whether the changes are severe enough and long term enough to actually influence people's vulnerability to disease. We are looking in to this," he said.
Phillip Hodson, a fellow of the British Association for Counselling and Psychotherapy said the research reinforced what is known.
"We all need some pressure in life. Stress is there to make sure you do your best in a challenging situation, whether it is running away from a sabre-toothed tiger or having to confront a difficult interview.
He said it made perfect sense that stress could boost the immune system.
"You come through [a stressful event] and you relax afterwards and, in a sense, you have had a work out. It's led to a greater sense of relief afterwards and your immune system has not had to work too hard for too long.
"What's damaging health wise is unrelieved stress," he said.