And it's because the milk contains immune cells—and not proteins as everyone had thought—that makes this protection for life possible.
Biologists had assumed that a baby got immunity from proteins, such as antibodies, in the milk, and that the immunity stopped the moment breastfeeding ended.
But researchers from Birmingham University have discovered that a mother's own immune response, possibly from an infection even before she became pregnant, was passed on in her milk to her child, and the protection is permanent. "Exposure to an infection before pregnancy can lead to a mother transferring long-term immune benefits to her offspring," said researcher Adam Cunningham.
The researchers based their findings on an experiment with helminths, a type of parasitic worm that infect the intestines. Laboratory mice that had been infected before pregnancy were passing on their immunity in their milk to their infants, ensuring their offspring had lifelong immunity against helminths.
This suggests that protection against other infections could also be passed on in breastmilk.