Australian study finds breastfeeding helps prevent breast cancer

Researchers in Australia have discovered how breastfeeding reshapes the immune system to provide long-term protection against breast cancer, particularly aggressive triple-negative types.

Published in Nature, the study provides a biological explanation for the protective effect of childbearing and shows how this has a lasting impact on a woman's immune system, according to a statement released Tuesday by Australia's Peter MacCallum Cancer Center (Peter Mac), which led the study.

"We found that women who have breastfed have more specialized immune cells, called CD8⁺ T cells, that 'live' in the breast tissue for decades after childbirth," said study lead author Professor Sherene Loi at Peter Mac.

"These cells act like local guards, ready to attack abnormal cells that might turn into cancer," she said, adding that this protection likely evolved to defend mothers during the vulnerable post-pregnancy period, and now also lowers breast cancer risk, especially aggressive triple-negative breast cancer.

Completing a full cycle of pregnancy, breastfeeding, and breast recovery caused these T cells to accumulate in the breast, Loi said, noting that their protective effect was confirmed in preclinical experiments.

"When breast cancer cells were introduced, the models with this reproductive history were far better at slowing or stopping tumor growth, but only if T cells were present," she said.

Data from over 1,000 breast cancer patients also indicated that women who breastfed had tumors with higher numbers of these protective T cells and better survival rates after diagnosis, researchers said.

While pregnancy-related hormonal changes were once believed to explain why childbearing lowers breast cancer risk, this research points instead to immune changes within the breast tissue as the key factor and suggests this could lead to entirely new breast cancer prevention and treatment.

As the second most diagnosed cancer in Australia, breast cancer is the most common cancer among women, with around 58 diagnoses daily in the country and a rising incidence in younger women, said the Peter Mac statement.