Early start: fetuses generate immune response to vaccination.

A fetus can manufacture immune cells and antibodies in direct response to vaccine given to the mother during pregnancy, according to researchers studying flu shots.

Scientists had already established that a pregnant woman can pass along certain antibodies to her fetus and that those immune proteins can protect a baby for up to 6 months after birth. Other studies had found that a fetus can muster an immune response to an infection contracted by the mother. But there had been little evidence indicating that a fetus can generate immunity to a vaccine, says study coauthor Rachel L. Miller, an immunologist at Columbia University College of Physicians and Surgeons.

Miller and her colleagues obtained blood samples from 70 pregnant women who had agreed to receive a flu shot about 7 months into their pregnancies. The Centers for Disease Control and Prevention recommend that women get flu shots in either the second or third trimester of pregnancy.

After each baby was born, the researchers analyzed samples of umbilical cord blood. Such samples provide a snapshot of the immune system that a baby had before birth.

The analyses showed that 28 of the 70 cord blood samples contained antibodies against a flu virus, the researchers report in the June 1 Journal of Clinical Investigation. The antibodies were too large to have passed through the placenta directly from the mother, Miller says.

In many cases, the fetus produced an antibody response even when the mother didn't, the researchers say. That suggests that the vaccine went from the mother's bloodstream through the placenta, spurring the antibody manufacture in the fetus, says Miller. The vaccine is made from a killed virus.

A separate analysis showed that some of the cord-blood samples also made immune system T cells that were specifically geared to fight the flu virus.

All these findings demonstrate that the fetus is "capable of generating its own immune response to vaccine" given to the mother, Miller says. However, researchers don't know whether the immunity is protective against disease.

"It's obviously important--if we choose to immunize mothers--to know to what degree we're immunizing the baby," says Thomas A. Platts-Mills, an allergist at the University of Virginia in Charlottesville.

Fetal immunity has long puzzled scienfists--particularly those who study allergy, which is a form of immunity run amok. Previous studies had suggested that a mother's exposure to allergens--ingested or inhaled--might affect her fetus' immune system.

"We think this [study] has implications for environment-related disease, including allergy," Miller says. She speculates that fetuses might respond to dust mite or cockroach allergens inhaled by the mother much as they react to flu vaccine.

Platts-Mills is skeptical of that idea because the new study" provides no evidence regarding response to allergens.

"They're two separate issues," he says. One difference, he explains, is that an injection of vaccine into a pregnant woman delivers a much greater immune affront to her fetus than, for instance, inhalation of a dust mite allergen does.