Patients who had more cells had better outcomes, particularly for aggressive types such as triple-negative breast cancer

In the 18th century, physicians noticed nuns had some of the highest rates of breast cancer. It was one of the earliest clues that led scientists to suspect that child-bearing and breastfeeding could protect against the disease.

Modern data has confirmed the centuries-old observation but the biological reasons behind it have remained unclear. Explanations have often focused on pregnancy-related hormonal changes, but research published Tuesday in Nature has found breastfeeding provides long-lasting immune protection.