Research suggests that when a pregnant woman is vaccinated against influenza, her baby can develop its own immune responses to the vaccination.

The finding is surprising because it is widely held that newborns and very young infants cannot mount a very effective immune defense against infection, but rely on protection acquired from their mother in the very early stages of life.

Dr. Rachel L. Miller of Columbia University College of Physicians and Surgeons, New York, and colleagues studied umbilical cord blood -- that is, blood circulating in the baby -- in more than 100 mostly Hispanic pregnant women after they were given a flu shot.

"Both antigen-specific T-cell and B-cell immune responses were detected," Miller told Reuters Health, referring to the two main types of immune system cells that guard the body against infections.

According to a report in the Journal of Clinical Investigation, the researchers detected anti-influenza antibodies in about 40 percent of cord blood specimens examined.

"The determination that the fetus can initiate B and T cell responses following exposure of the mother during pregnancy has wide implications for multiple diseases," write the researchers.

"Our findings may have implications for how early the immune response to environmental exposures may occur," Miller said.

As she and her colleagues note in their article, it looks like the immune system of newborns "is not deficient or incompetent but, rather, capable of responding to environmental exposures."