A new romance can cause women to produce a specific kind of protein typically used to combat infection - suggesting being 'love sick' is more than just an emotion.
Scientists at the University of California in Los Angeles, who carried out the research, suggest these changes mean love could one day be 'measured'.
The team took blood samples from 47 women over the course of a two-year period as they entered new romantic relationships.
Their aim was to monitor how falling and being in love affects genes governing the immune system.
Researchers found that new love activates genes to produce interferon, which is a protein usually released to combat viruses.
'New romantic love is accompanied not only by psychological changes, but physiological changes as well,' scientists noted in the report, published in the journal Psychoneuroendocrinology.
The report continued: 'These findings are consistent with a selective up-regulation of innate immune responses to viral infections... and provide insight into the immuno-regulatory correlates of one of the keystone experiences in human life.'
The study set out to determine the impact romantic love has on the human genetic function.
Findings suggested that it may also be possible to test to see if people really are in love and whether that emotion gradually wades over a period of time.
'Some research suggests that psychological changes associated with romantic love may be attenuated as the relationship matures,' the experts said.
'The biological correlates of love might abate with the maturation of a longer-term more stable mate bond.'
In those cases where the flame had started to burn out, researchers found evidence of women's interferon levels decreasing.
'Falling out of love was associated with a reduction in interferon-related gene expression,' the researchers found.
Scientists are yet to examine whether the experience is the same for men.
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