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Young gun.
Even fetuses appear to be able to mount complex immune responses.

Flu-Fighting Fetuses

Via the placenta, a newborn baby receives a 6-month supply of antibodies from its mother, arming it against a world chock-full of allergy-causing particles and viruses. But it turns out that the baby may have been preparing its immune system for battle well before birth. New research indicates that developing fetuses are able to mount their own specific immune response to flu vaccines received by their mother. The findings could help end a debate over just how complex a fetus's immune system is.

A fetus contains many kinds of immune cells, yet most immunologists believe those cells are too immature to target specific allergy-causing molecules, or antigens. That's because no antigen-specific antibodies had ever been found in umbilical cord blood. Instead, immunologists assumed that a fetus could only launch general attacks on infections, while relying on the mother's immune system for antigen-specific responses. But Rachel Miller, an allergist and immunologist at Columbia University, believed the fetal immune system was more advanced than researchers were giving it credit for.

Miller and colleagues studied a group of 126 women who received a flu vaccine during pregnancy. When those women gave birth, the researchers took samples of each newborn's cord blood. They collected 70 usable samples and used a technique that had never been applied to cord blood to look at the immune response at the cellular level. That way they could identify, cell by cell, whether a fetus had produced specific antibodies in response to the flu vaccine. They found that 40% of the samples contained antigen-specific T and B immune cells and antibodies. Miller says it's unclear why only some of the fetuses exhibited an immune response, but she says what's significant is that the flu-specific antibodies were there at all. Some were IgM antibodies, which are too large to pass through the placenta from mother to child, meaning they were undoubtedly produced by the fetus. Far from being defenseless, the fetal immune system is quite capable of responding to infection, the team reports today in the Journal of Clinical Investigation.

The results help confirm the long-contentious theory that fetuses can mount specific immune responses, and they are "a real bonus" to the field, says Aimen Shaaban, an immunologist at the University of Wisconsin, Madison. Still, he notes that the vaccinations were given in the third trimester, by which point the fetal immune system may have had time to become more complex. If the work is repeated, Shaaban says, it would be good to look at fetuses in earlier trimesters to get a better idea of when, exactly, these advanced immune system responses begin to kick in.

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