Breastfeeding Has Long Been Linked to Reducing Cancer Risk. Scientists May Have Just Discovered Why

Patients with a history of breastfeeding showed higher levels of specialized immune cells in their breasts that may protect against malignant cancers

Marta Hill | Freelance contributor

October 24, 2025

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Scientists have long suspected that breastfeeding helps protect against breast cancer. Epidemiological data and advancing research over the years have built on the theory—and scientists may have just found one underlying explanation.

In a study published on October 20 in Nature, scientists analyzed human breast tissue, looked at immune responses in mice and examined clinical data from more than 1,000 people with triple-negative breast cancer to unpack the immune effects of breastfeeding.

In the first phase of the study, the research team looked at a diverse group of 260 people who had undergone preventative mastectomies or breast reductions. The researchers found more of a special type of immune cells, called CD8+ T cells, in those who had given birth.

“These cells act like local guards, ready to attack abnormal cells that might turn into cancer,” study author Sherene Loi, an oncologist and breast cancer specialist at the Peter MacCallum Cancer Center in Victoria, Australia, says in a statement.

Those specialized immune cells stuck around for up to 50 years in some people.

“This study shows that having babies and breastfeeding causes long-lasting changes in immune cells that could help protect the breast from cancer,” Wendy Ingman, a breast health researcher at the University of Adelaide Medical School, tells Natasha May at the Guardian. Ingman was not involved in the new study. “I’m hopeful that this type of research will lead to new approaches to reduce women’s breast cancer risk.”

Taking the work a step further, Loi and her research team looked at mice, some of which had completed a full cycle of pregnancy, lactation and mammary recovery and some that had not.

This allowed them to compare specialized T cell levels across the populations. The animal findings lined up with the ones in humans—mice that had been pregnant and lactated had more specialized T cells in their mammary tissue than those who had not.

To test if the immune cells guarded against breast cancer, Loi and her team implanted an aggressive form of breast cancer cells into the mice. Tumors grew far slower in mice that had completed a full cycle of lactation.

“The key take-home messages [are] that pregnancy and breastfeeding will leave behind long-lived protective immune cells in the breast and the body, and these cells help to reduce risk and improve defense against breast cancer, particularly triple-negative breast cancer, but potentially other cancers as well as disease,” Loi tells the Guardian.

When researchers depleted the immune cells in the mice, the tumors grew quickly.

“Immunity was both in the breast and systemic. So lactation does actually change the whole body’s immunity in these mice models,” Loi tells Starre Vartan at Nature.

Bringing the research back to humans, Loi and her colleagues looked at clinical data from more than 1,000 patients with triple-negative breast cancer after pregnancy. Those who had breastfed had a higher density of the specialized immune cells.

“This suggests there was ongoing immune activation and regulation from the body against their breast cancer,” Loi tells Larissa Fedunik at NewScientist.

This research does not answer every question surrounding the immune benefits of breastfeeding, Julia Ransohoff, an oncologist and breast cancer researcher at Stanford University, tells Nature. It is still unclear how these immune cells persist for so many years and how they relate to hormone-sensitive cancers.

“It lays the foundation for future work that might explain how the CD8+ T cells retain a ‘memory’ of breastfeeding,” Daniel Gray, an immunologist at the Walter and Eliza Hall Institute of Medical Research in Victoria, Australia, tells NewScientist.

The study focuses specifically on triple-negative breast cancer, making it unclear if the protective effect will extend to other types of cancer, Nicholas Huntington, head of the Cancer Immunotherapy Laboratory at Monash University in Australia, tells Olivia Willis at the Australian Broadcasting Corporation.

Huntington, who was not involved in the research, tells the outlet, “What this study enables us to do is get a better understanding of the immune cell types, some of the growth factors and signaling molecules that might be more important to target with emerging therapies.” 

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Marta Hill is a science journalist based in New York City and a former intern at Smithsonian magazine. She is a part of the 2025 class of New York University’s Science, Health and Environmental Reporting Program. Her work has appeared in the Transmitter, Eos, Boston.com and more.