Cancer Research UK scientists at The University of Manchester found that chemical signals produced by a type of immune cell, called macrophages, also act as a survival signal for melanoma cells.
When the researchers blocked the macrophages' ability to make this signal - called TNF alpha - melanoma tumours were much smaller and easier to treat.
When melanoma patients are given chemotherapy or radiotherapy it causes inflammation, increasing the number of macrophages in the body - and raising the levels of TNF alpha.
"This discovery shows that immune cells can actually help melanoma cells to survive. Particularly when patients are receiving treatment, the immune cells produce more of the survival signal, which makes treatment less effective," Dr Claudia Wellbrock, study author and Cancer Research UK scientist at The University of Manchester said.
"So combining standard treatment with immunotherapy at the same time could potentially provide more long-lasting and effective treatments to increase survival," said Wellbrock.
Melanoma is the most deadly form of skin cancer with around 13,300 people diagnosed in the UK each year. Rates of the disease have increased more than fivefold since the mid 1970s, researchers said.
"Drugs which block this 'survival signal' have already been developed and using these along with standard treatment may be a promising new approach for melanoma patients," said Marais.
The research was published in the journal Cancer Discovery.