The findings, published in Nature and announced Tuesday by the Peter MacCallum Cancer Centre (Peter Mac), provide the strongest biological evidence yet for the long-observed link between childbearing, breastfeeding, and reduced breast cancer risk.
According to the study’s lead author, Professor Sherene Loi, women who breastfeed develop a population of specialised immune cells known as CD8⁺ T cells, which remain embedded in breast tissue for decades after childbirth.
“These cells act like local guards, ready to attack abnormal cells that might turn into cancer,” Loi explained, adding that the protective mechanism likely evolved to defend mothers during the vulnerable post-pregnancy period.
The research found that completing the full reproductive cycle — pregnancy, breastfeeding, and breast tissue recovery — stimulates these T cells to accumulate in the breast, creating a long-term surveillance system against potential tumour growth.
In laboratory experiments, breast tissue models that had undergone this reproductive cycle were far more effective at slowing or stopping tumour development — but only when the protective T cells were present.
Data from more than 1,000 breast cancer patients further supported the findings. Women who had breastfed not only showed higher levels of these immune cells in their tumours but also had better survival outcomes after diagnosis.
The study challenges the long-held belief that hormonal changes during pregnancy are the primary factor behind reduced breast cancer risk. Instead, it highlights immune system reprogramming within the breast as a potentially more critical driver — one that could inspire new strategies for cancer prevention and immunotherapy.
As breast cancer remains the most common cancer among Australian women — with about 58 new cases diagnosed daily — researchers say these insights could have major implications for understanding and combating the disease, particularly the aggressive triple-negative subtype.
Peter Mac noted that breast cancer incidence continues to rise among younger women, underscoring the importance of further research into reproductive and immune factors that may influence long-term cancer risk.