A type of normal cell often found in cancerous growths may be the reason for the failure of anti-cancer vaccines.
Stromal cells may act to protect tumours from the body's immune system, a study in the journal Science reports.
Cambridge University scientists eliminated some of these cells in mice, and shrank their lung tumours.
Cancer Research UK said the study offered "exciting clues" to how cancer recruited healthy cells, and how to prevent that happening.
Tumours are not just made up of cancer cells - often these are interspersed with normal cells carrying on with their normal functions.
Stromal cells are part of the body's connective tissue, helping provide fibres and structures to support other tissues and cells.
The Cambridge study suggests that, in some tumours at least, their activity is holding back the immune system from launching attacks which could shrink or destroy tumours.
This is particularly relevant for vaccines used as treatments once a patient is diagnosed with cancer, which aim to boost this immune response.
The failure of these vaccines to significantly affect tumours, even though they provoke an immune response in the body, has been a mystery to scientists.
The stromal cells in question have a protein on their surface called fibroblast activation protein alpha (FAP), which normally has a role in wound healing.
Here, the researchers suspected it was suppressing the immune system in and around the tumour.
To test this, they created a mouse in which FAP-producing cells could be eliminated.
When this happened in animals with well-established lung tumours, the cancer rapidly shrunk.
Only 2% of the cells within these tumours actually produced FAP, so the scientists are convinced that they had a far wider role in protecting the tumour from the immune system.
Professor Douglas Fearon said that identifying these cells, found in many human cancers, including breast and colon cancers, was an "important step".
He said: "Further studying how these cells exert their effects may contribute to improved immunological therapies by allowing us to remove a barrier that the cancer has constructed.
"These studies are in the mouse and although there is much overlap between the mouse and human immune systems, we will not know the relevance of these findings in humans until we are able to interrupt the function of the tumour stromal cells expressing FAP in patients with cancer."
Dr Claire Knight, from Cancer Research UK, said the study offered "exciting clues" as to how tumours could recruit healthy cells to help them evade the immune system.
She added: "Immunotherapy is a promising area of research, but its use for treating cancer is limited because tumours can hide from the immune system.
"Finding ways to target these newly-recruited healthy cells could help to make immunotherapy more effective in the future, although more research is needed before this becomes a reality."