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.
Australia’s Peter MacCallum Cancer Centre (Peter Mac), which led the study, made this known vis a statement issued on Tuesday, October 21, 2025.
Prof. Sherene Loi, who is the study lead author at Peter Mac, said: “We found that women who have breastfed have more specialised immune cells, called CD8⁺ T cells, that ‘live’ in the breast tissue for decades after childbirth.
“These cells act like local guards, ready to attack abnormal cells that might turn into cancer.”
She added 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.
Loi said completing a full cycle of pregnancy, breastfeeding, and breast recovery caused these T cells to accumulate in the breast, adding that their protective effect was confirmed in preclinical experiments.
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“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.
Although it was once thought that pregnancy-related hormonal changes were a major factor childbearing lowers breast cancer risk, Chinese news agency Xinhua reported.
The 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, they said.
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.
It is a rising incidence in younger women, said the Peter Mac statement.
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