Researchers in Australia have uncovered how breastfeeding reshapes the immune system to provide long-term protection against breast cancer, particularly the aggressive triple-negative type.
The groundbreaking study, published in Nature, offers a biological explanation for the protective effect of childbearing and demonstrates how it leaves a lasting imprint on a woman’s immune system.
According to a statement released on Tuesday by Australia’s Peter MacCallum Cancer Centre (Peter Mac), which led the research, women who breastfeed develop specialized immune cells that remain active in breast tissue for decades after childbirth.
“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 Prof. Sherene Loi, lead author of the study and researcher at Peter Mac.
“These cells act like local guards, ready to attack abnormal cells that might turn into cancer,” she explained.
Prof. Loi added that this protection likely evolved to defend mothers during the vulnerable post-pregnancy period but now also helps reduce the risk of breast cancer, particularly the aggressive triple-negative subtype.
She noted that completing a full cycle of pregnancy, breastfeeding, and breast recovery caused these T cells to accumulate in breast tissue. 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 showed that women who had breastfed developed tumors with higher numbers of these protective T cells and had better survival rates after diagnosis.
While pregnancy-related hormonal changes were previously thought to be the main reason childbearing lowers breast cancer risk, the researchers said this study points instead to immune changes within breast tissue as the key factor.
They added that the findings could pave the way for entirely new strategies in breast cancer prevention and treatment.
Breast cancer is the most common cancer among women and the second most diagnosed cancer overall in Australia, with around 58 new cases reported daily. The Peter Mac statement also noted a rising incidence among younger women.
