We already knew that breastfeeding reduces the risk of breast cancer. Epidemiological data have been saying this for years: approximately 4,3% less likely for each year of breastfeeding. But the mechanism remained unclear, a correlation without a precise cellular explanation. Now a group of Australian researchers has brought the picture into focus.
Researchers analyzed breast tissue from 260 women of different ages and ethnicities, and discovered something unexpected: women who have breastfed harbor a particular type of immune cells in their breasts, the CD8+ T lymphocytes, which remain there for life. They don't disappear after weaning. They remain in place, like sentinels ready to attack abnormal cells. And they work.
The guards who remain on patrol
Sherene Loi, oncologist of the Peter MacCallum Cancer Centre of Melbourne, coordinated the study published in NatureThe team examined breast tissue from women between the ages of 20 and 70, none of whom had a previous diagnosis of breast cancer. What they found is a population of CD8+ T cells that behave like permanent residents of the tissueThey don't move. They just sit there, anchored. According to the published results, these cells They can remain active in the breast for up to 50 years after the last breastfeeding.
“They act as local guards, ready to attack abnormal cells that could turn into cancer,” Loi explains.
Interestingly, this accumulation doesn't just occur during pregnancy. It takes the entire cycle: pregnancy, lactation, and the recovery period after weaning. The researchers verified this in mice, where they could monitor every stage of the process. Animals that completed the entire cycle showed a significant increase in T cells in their mammary tissue. Those whose pups were removed immediately after birth did not.
Breastfeeding: Slower tumor growth, longer survival
The researchers didn't stop at observation. They tested the hypothesis by implanting cells of triple-negative cancer, one of the most aggressive forms of breast cancer, in mice that had completed the lactation cycle. Result: tumor growth was dramatically slowed compared to control miceBut when they artificially eliminated CD8+ T cells from these animals, the tumors quickly rebounded. The connection is direct.
In a subsequent phase, the researchers analyzed clinical data from more than 1.000 women diagnosed with triple-negative cancer who had had at least one full-term pregnancy. Women who had breastfed showed tumors with a higher density of CD8+ T cells, a sign that the immune system was still active against the cancer. And overall survival was significantly longer, even after adjusting for other risk factors such as age.
The accumulation of T cells in the breast during breastfeeding is not accidental. It serves to prevent infections that could lead to mastite, a painful inflammation of the breast tissue. But this temporary defense leaves a positive side effect: a permanent immune memory that continues to monitor the tissue for the woman's entire life.
The pregnancy issue
The relationship between pregnancy and breast cancer is more complicated. According to the Ministry of HealthEarly pregnancy reduces the risk, but only if it occurs at a young age. A late pregnancy, however, can temporarily increase it. Breastfeeding, on the other hand, has a more consistent protective effect and is less dependent on age. According to WHOApproximately 20.000 cases of breast cancer are prevented each year by breastfeeding. If this practice were improved and better supported, another 20.000 cases could be prevented.
Loi emphasizes that the choice to breastfeed remains individual and not always possible. "This doesn't mean that breastfeeding completely prevents breast cancer," she clarifies. But the research provides a window into how the body builds long-term defenses and could suggest future strategies for those who can't or don't want to breastfeed.
Breastfeeding and breast cancer: prevention on multiple fronts
Daniel Gray, researcher of Walter and Eliza Hall Institute of Medical Research Victoria, commented on the study, calling it robust for analyzing multiple groups of women. "It lays the foundation for understanding how CD8+ T cells maintain a 'memory' of breastfeeding," she says.
The next question is: can we replicate this effect in the laboratory?
The idea is to develop approaches that mimic the protective effect of breastfeeding without requiring pregnancy. It could be useful for women with BRCA genetic mutations or other high-risk factors. But we're still far from clinical applications. For now, research tells us that the body has much more sophisticated defense strategies than we thought. And that breastfeeding isn't just nourishment for the newborn: it's a long-term biological investment for the mother.
Who knows if we will be able to exploit this knowledge to protect even those who have never breastfed, or if it will remain an evolutionary advantage accessible only to those who complete that specific cycle.
Meanwhile, we know those T cells remain there, on guard. For fifty years. Or more.
