Women are more likely to survive aggressive forms of breast cancer if they have a specific type of immune cell near their tumour, a study has found.
The "killer" T-cells specialise in destroying infected or damaged cells, such as cancer cells, by blasting them with toxic proteins.
Scientists found that breast cancer patients with killer T-cells around their tumours were 10 per cent more likely to live for five years or more.
The association was seen in women with non-hormone sensitive breast cancer and cancers marked by especially active HER2 genes.
Both are aggressive forms of the disease. The presence of killer T-cells did not affect the survival of women with "ER-positive" tumours sensitive to the hormone oestrogen, which make up the majority of cases.
Lead researcher Dr Raza Ali, a clinical lecturer at the Cancer Research UK Cambridge Institute, said: "Cancer often finds ways to escape the immune system, but helping immune cells to recognise cancer as a threat - and attack it - provides a promising and powerful avenue for new treatments.
"We've shown that women who have killer T-cells present at the site of their tumour are likely to live longer.
"This important insight could help doctors personalise a woman's treatment based on her immunological profile and also suggests that new treatments should harness the immune system to fight cancer."
Immune cells were counted and analysed from samples collected from 12,439 breast cancer patients taking part in four different studies in England and Canada.
The findings also showed that chemotherapy treatments such as doxorubicin could be enhanced by the presence of T-cells.
Professor Carlos Caldas, senior group leader at the Cancer Research UK Cambridge Institute, said: "The more we learn about how precisely the immune system interacts with breast cancer the better we are able to fine tune the treatments we give to patients - and the sooner we can save more lives."
Professor Peter Johnson, the charity's chief clinician, said: "This research highlights the great strides we are making in understanding the complex interplay between cancer and the body's immune system. These studies are key to informing how we are best able to treat patients in the clinic and to design better drugs that make the best use of the body's own defences."
The research is published in the journal Annals of Oncology.
