Depression is a physical illness that could be treated with anti-inflammatory drugs, according to a Cambridge University professor.
An overactive immune system may trigger the mental health condition by causing widespread inflammation that leads to feelings of hopelessness and unhappiness, experts believe.
The immune system may fail to 'switch off' after an illness or traumatic event, they add.
Past research has shown people who suffer severe emotional trauma have signs of inflammation, suggesting their immune system is constantly 'fired-up'.
Around one in 13 people in the UK suffer from depression, which is largely treated by restoring feel-good chemicals in the brain.
Could the Illegal Party Drug Ketamine Ease Depression?
The illegal party drug ketamine could ease depression, research suggested last month.
Patients who respond to the horse tranquilizer have greater overall activity levels days after its infusion, a study found.
Depressive episodes are partially characterized by a reduced interest in activities.
Responsive patients also sleep better the following day, the research adds.
Ketamine is a Schedule III drug in the US and a class B in the UK. It can legally be prescribed by doctors as a general anesthetic in humans and animals.
'Robust association between inflammation and depressive symptoms'
Professor Ed Bullmore, head of the department of psychiatry at the University of Cambridge, said: 'In relation to mood, beyond reasonable doubt, there is a very robust association between inflammation and depressive symptoms.
'In experimental medicine studies if you treat a healthy individual with an inflammatory drug, like interferon, a substantial percentage of those people will become depressed,' The Telegraph reported.
Researchers from Cambridge University and the Wellcome Trust are hoping to initiate studies next year investigating anti-inflammatory drugs' efficacy in depression.
Immune system triggers inflammation when under threat
When the immune system suspects a threat, it triggers inflammation, leading to changes in the body, such as increased red blood cell counts, in preparation to heal a wound.
Until recently, such a process was denied as a cause of depression as scientists believed the brain and the immune system operated separately.
Yet, recent studies show a link between nerves in the brain and immune function.
Figures have also revealed around 60 per cent of people who visit a doctor with chest pain are actually suffering from anxiety, while approximately 30 per cent of those with conditions such as arthritis have the mental health condition, which is four times high than the population average.
Professor Sir Robert Lechler, president of the Academy of Medical Sciences, said: 'You’re not just a little bit miserable if you’ve got a long-term condition, there is a real mechanistic connection between the mind, the nervous system and the immune system.'
Experts add the outdated approach of separating mental and physical health conditions is holding back medical advances.
This comes after research from North Carolina State University revealed one in five women with postpartum depression keep their anxieties to themselves.
Of these, around half of women claim to have at least one barrier that makes asking for help 'extremely difficult' or 'impossible'.