Immune responses to flu vaccines differ with age. (Rogelio V. Solis/Associated Press)

Our immune systems respond differently to flu vaccines as we age, a small U.S. study suggests.

The body's antibodies are often compared to foot soldiers defending against foreign invaders.

In Wednesday's issue of the journal Science Translational Medicine, U.S. and Chinese researchers described using DNA sequencing technology to count antibodies during the peak immune response in 17 people ranging in age from eight to 100.

They found that both the number of antibodies and their diversity decreased with age, meaning the immune system seems to have fewer weapons for the battle.

The research offers "direct insight into the effects of vaccination and provides a detailed molecular portrait of age-related effects," Stephen Quake of the Broad Institute of MIT and Harvard in Cambridge, Mass. and his co-authors concluded in the study.

In the study, they used seasonal flu vaccines from 2009 and 2010 and measured changes in antibody "repertoire."

The largest antibodies, called immunoglobulin M or IgM that act as first responders, decreased with age.

That finding, the researchers said, is consistent with current thinking that the elderly are more likely to use another type of antibody that offers a longer lived immune response after exposure to infection.

But it's too early to say if the differences make vaccines more or less effective.

The study also does not suggest that the elderly should stop getting flu shots but that the oldest individuals should reduce activities that could increase their exposure to flu and get medical advice at the first signs of infection, the researchers said in a journal release.

The study was funded by U.S. National Institutes of Health.