A  recent study has found that a brief session of muscle-building exercise before receiving a flu shot can enhance a person’s immune response. The immune system’s reaction differs markedly between men and women, however, and exactly how exercise influences the process remains unclear.

These findings help confirm an earlier study by Edwards’ team, which showed for the first time that the acute stress of exercise can increase antibody production in humans. The study appeared in the February 2007 issue of Brain, Behavior, and Immunity.

The researchers focused on eccentric exercise, which occurs when a person lowers a heavy weight. This type of exertion consistently causes a local inflammatory response that includes the arrival of key immune cells.

In this latest randomized controlled trial, 60 healthy college students either rested quietly or performed eccentric exercise of the upper arm for about 25 minutes. Six hours later, all participants received an injection of flu vaccine, which was donated by GlaxoSmithKline, UK.

Participants had their blood tested repeatedly over the next five months to measure the cellular immune response as well as the longer-lasting development of antibodies to the viruses in the vaccine.

The cell-mediated response remained unchanged in female participants, while it increased among males. The antibody response was enhanced in women, but reduced in men.

While these results are less clear than researchers had hoped, Edwards said, they confirm that the inflammatory response to acute stress plays a key role in the immune system.

Another key question is whether the immune responses measured in the latest study have clinical significance, according to Anna Marsland, Ph.D., of the University of Pittsburgh: “Is somebody better able to protect themselves from flu if they have slightly higher antibody levels?” For now, there’s no evidence one way or the other.

Nevertheless, said Edwards, “If you manage to fit in doing some exercises before you get your flu shot, that certainly could benefit you in many ways and might well benefit your vaccination response.”

Reference: Edwards KM, et al. Eccentric exercise as an adjuvant to influenza vaccination in humans. Brain, Behavior, and Immunity, 21(2), 2007.