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.

Prof Sherene Loi, a clinician scientist at the Peter MacCallum Cancer Centre in Melbourne, said the most sophisticated level of the immune system, called the adaptive immune system, involves T-cells which react against particular viruses or bacteria – and also against can

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