A vital part of the immune system is put to sleep by cancer cells, say researchers investigating how it spreads through the body.

The study carried out by Stanford University has found that the so-called "killer" T-cells, which are normally on the front line in the fight against disease, simply do not mount their normal defence when faced with a cancer cell.

Researcher Mark Davis said: "Normally T cells are very responsive, even small amounts of antigen get them going."

Cancer cells to blame

He concluded that the cancer cell itself was responsible for disabling the T cell attack, and are hoping to investigate exactly why.

His colleague Peter Lee said: "What we've found is that cancer cells don't just sit there and wait to be destroyed.

"They fight back, one thing they do is turn off the cells that are trying to destroy them."

He added: "In patients who get cancer, the cancer is winning the battle."

Problems for vaccine research

He said that his results, which are published in the journal Nature Medicine, could make a search for a cancer vaccine far more difficult.

Researchers in that area were hoping to harness the body's own immune system against the cancer cells.

The research group looked at blood samples from 11 patients with skin cancer, and found that only T cells that would normally target cancer cells were inactive.

Other T cells which might tackle viruses like the common cold or influenza behaved normally, meaning that the patients' overall immune system was working.

Each T cell is set up to tackle a single sort of foreign particle, and the need to produce a supply from scratch to tackle an infection explains why it takes the body so long to respond to 'flu or a cold.

When the infection is conquered, the template for building the necessary T cells is retained by the body, making it easy to fight it off again - which is why the body becomes immune to diseases when it has defeated them.