Breast milk does more to protect baby girls from respiratory infections than it does for boys, researchers said.

A team from Johns Hopkins Children's Center followed 119 premature babies in Buenos Aires for a year. A news release on the study said it casts doubt on the idea that breast milk passes immune system properties from mothers to babies.

Girls who were fed formula had the highest rate of infections. They were eight times more likely than breast-fed girls to have problems.

Breast-feeding did not appear to affect the number of infections but rather their severity and the need for hospitalization, meaning that breast milk does not prevent a baby from getting an infection but helps a baby cope with an infection better, according to the release.

Dr. Fernando Polack said it seems that rather than protecting against specific diseases, breast milk turns switches on a protective system. But he did not know why it appears to be easier to do that in girls than boys.

The findings, researchers say, are particularly important for health care in developing countries, where antibiotics and other treatments are scarce and where an estimated one-fourth of premature babies end up in the hospital with severe respiratory infections.