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Breastfeeding Promotes Cells That Reduce Breast Cancer Risk

    A graphic depicting the cycle of breastfeeding to indicate the newfound relationship between breastfeeding and a reduction in breast cancer risk.

    Written by: Alla Katsnelson, Ph.D | Issue # 130 | 2026

    • CD8+ T cells are enriched in healthy women and mice who have given birth compared to those who have not. Mice who completed the natural lactation cycle had even higher CD8+ levels and less tumor growth in response to cancer cells introduced into their mammary tissue.
    • In a group of women with triple negative breast cancer, those who had given birth to at least one child and had breastfed had better survival rates. 
    • Understanding the underlying biology could point to prevention or treatment strategies for breast cancer.

    Breastfeeding protects against breast cancer – researchers have known that’s true for more than a century. But how that protection is conferred has been something of a mystery. 

    A new study published in October 2025 provides an eye-opening start to an answer. Women who had children and breastfed had higher numbers of specialized immune cells called CD8+ T cells [1]. These cells, which are known to fend off breast cancer [2], stick around for decades after, both in breast tissue and throughout the body.

    “Our research opens up a new area of biology,” said Sherene Loi, MD, PhD, associate professor at Peter MacCallum Cancer Centre in Melbourne, Australia, who co-led the study, in a press conference about the work at the European Society for Medical Oncology annual meeting, according to Medscape. “Now we can potentially move to understanding how we can harness that for women who do not breastfeed or who remain nulliparous.”

    One early hint that some aspect of having children protected women from breast cancer came in 1713 when Italian physician Bernardino Ramazzini noticed that nuns experience breast cancer at unusually high rates [3]. In fact, although the term is outdated now, breast cancer was long referred to as ‘nun’s disease.’ It’s thought that the dynamic cellular changes that occur within the mammary gland during pregnancy, lactation, and the tissue’s return to a pre-pregnancy state (a process called involution) reduce the tissue’s susceptibility to tumors [4]. 

    Studies suggest that having children is associated with a reduction in a type of breast cancer called hormone receptor positive breast cancer, in which cancer cells carry estrogen and progesterone receptors. Meanwhile, breastfeeding is associated with a reduction in another type of breast cancer, called triple negative breast cancer, in which the cancer cells lack receptors for estrogen, progesterone and a protein called HER2. This subtype of the disease tends to affect younger women and to be more aggressive and harder to treat [5].

    Loi and her colleagues homed in on the biological underpinnings of these observations by studying both women and lab mice. 

    First, the researchers compared breast tissue from 170 healthy women with a typical risk of cancer and 65 women with an elevated risk of cancer. The tissue came from either breast reduction surgeries or preventative mastectomies. In both groups, women who had given birth to at least one child had significantly more CD8+ cells than women who had never given birth. This remained true several decades  after giving birth.

    They then examined the protective role of CD8+ cells in mice. Mice who completed the natural cycle of lactation, which is 21 days, had higher levels of CD8+ cells than did mice who were allowed to nurse their litters for only 12-24 hours or who never had babies at all. Additionally, when the researchers introduced cancer cells into the mammary tissue of these three groups, the group that experienced the usual cycle of lactation and involution had less tumor growth. But those mice had less durable tumor protection when their CD8+ cells were depleted. Blocking these immune cells’ deployment from the thymus, where they are produced, to the mammary tissue had the same effect. 

    Finally, the researchers looked at data from about 1000 women under 40 who had given birth to  at least one child and had triple negative breast cancer. They found that tumors of  women who breastfed had higher numbers of CD8+ T cells, and in one group they examined, these women had better disease outcomes. 

    It’s still unknown how exactly CD8+ T cells provide protection, but identifying the underlying biological processes involved may point to new immunotherapies or other approaches for preventing or treating breast cancer by scaling up or mimicking these immune cells’ activity. 

    References

    1. Virassamy B, Caramia F, Savas P, Harris MA, Pan JW, Wang J, Brown E, O’Malley MMR, van Geelen CT, Hun M, Burn TN, Sant S, Ballan JD, Kay J, Lara Gonzalez LE, Clarke K, Aw Yeang HX, Idrizi R, Jana M, Challice DJ, Salgado R, Thorne H; kConFab consortium; Poliness C, Nightingale S, Teo SH, Speed TP, Visvader J, Neeson PJ, Darcy PK, Mackay LK, Loi S. Parity and lactation induce T-cell-mediated breast cancer protection. Nature. 2025 Oct 20. doi:10.1038/s41586-025-09713-5. Epub ahead of print. 
    2. Islami F, Liu Y, Jemal A, Zhou J, Weiderpass E, Colditz G, Boffetta P, Weiss M. Breastfeeding and breast cancer risk by receptor status–a systematic review and meta-analysis. Ann Oncol. 2015;26(12):2398-407.
    3. Britt K, Short R. The plight of nuns: hazards of nulliparity. Lancet. 2012;379(9834):2322-3. 
    4. Russo J, Moral R, Balogh GA, Mailo D, Russo IH. The protective role of pregnancy in breast cancer. Breast Cancer Res. 2005;7(3):131-42. 
    5. Li Y, Zhang H, Merkher Y, Chen L, Liu N, Leonov S, Chen Y. Recent advances in therapeutic strategies for triple-negative breast cancer. J Hematol Oncol. 2022;15(1):121.