Now, researchers are starting to understand how that transfer happens on a molecular level.

A study out today shows how antibody-producing cells get to the mammary glands.

"We discovered the off-ramp that directs these white blood cells to the mammary gland where they then make antibodies that are passed into the . . .intestines of the nursing infant," said Eric Wilson, a Brigham Young University microbiologist who led the research.

Years before a woman is pregnant, cells that produce antibodies against intestinal infections end up in her intestines. When she begins to nurse, some of the cells head to the mammary glands.

According to the Food and Drug Administration, 80 percent of the cells in breast milk kill bacteria, fungi and viruses. That's why breast-fed infants have lower rates of hospital admissions, ear infections, diarrhea, rashes and allergies than babies fed infant formula.

The research team identified a molecule dubbed CCR10 as the one that signals the cells to head to the mammaries. Wilson said they don't yet know what causes the body to produce the molecule.

Wilson said the research is important to understanding basic biology. It also could have implications, far in the future, for developing vaccines that could pass immunity from mothers to their infants.

And, by understanding what makes cells go to certain places, researchers could use the information to block cells from traveling to other locations - to prevent white blood cells from accumulating in the lining of the intestines, causing Crohn's disease, for example.

The study was funded by a National Institutes of Health grant to look into how white blood cells migrate to the mammaries. Wilson said he is looking for other molecules that play a role.

The research - which involved creating mice without the CCR10 molecule - included researchers from Harvard and Stanford. It was published in the Journal of Immunology.