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.