Washington, Aug 18 - Immune cells or macrophages that protect us against invaders or disease, turn traitors and actually help some of the deadliest cancers to progress, says a new study.

Albert Einstein College cancer researcher Jeffrey W. Pollard and seven colleagues analysed the movement of breast cancer cells in mice to show that a distinct population of macrophages helps malignant cells set up shop at distant sites.

This process, known as metastasis, is the main reason cancer patients die. Pollard and his colleagues believe that their discovery offers a potentially useful new target for anti-cancer therapy.

What they've found is a vulnerable step in the cancer process that might be blocked by drug treatments. In three different ways, the scientists showed that metastatic tumour growth is inhibited if these unusual macrophages are killed.

They also showed that even after breast cancer cells have lodged in the animals' lungs and started aggressive growth, erasing the special macrophages dramatically slowed growth of the metastasized tumors.