The connection between exercise and inflammation has interested researchers since a study showed a spike of white cells in the blood of Boston marathon runners following the race.

A new Harvard Medical School study, published in the journal Science Immunology offers a molecular explanation, suggesting that the beneficial effects of exercise may be driven in part by the immune system.

Researchers found that muscle inflammation caused by exertion mobilises inflammation-countering T cells (Tregs), which enhance the muscles' ability to use energy as fuel and improve overall exercise endurance.

Long known for their role in countering the inflammation linked to autoimmune diseases, Tregs were identified as key players in the body's immune responses during exercise.

"The immune system – the T cells in particular – have a broad impact on tissue health that goes beyond protection against pathogens and controlling cancer,” said Diane Mathis, professor of immunology at Harvard Medical School, “our study demonstrates that the immune system experts powerful effects inside the muscle during exercise.”

The findings come amid intensifying efforts to understand the molecular underpinnings of exercises and untangling the immune system's involvement in this process is one aspect of these research efforts.

"We've known for a long time that physical exertion causes inflammation, but we don't fully understand the immune processes involved," said study author Kent Langston, postdoctoral researcher in the Mathis lab. "Our study shows, at very high resolution, what T cells do at the site where exercise occurs – in the muscle."

Treg activity had a positive impact on exercise-induced inflammation in cases of both regular and one-off exercise, however, the metabolic and performance benefits of this immune system response were observed only in the regular exercisers.

Sedentary subjects did not experience either benefit.

The study findings provide a glimpse into the cellular inner workings behind exercise's anti-inflammatory effects and underscore its importance in harnessing the body's immune defenses, researchers said.

"Our work suggests that with exercise, we have a natural way to boost the body's immune responses to reduce inflammation," Mathis said. "We've only looked in the muscle, but it's possible that exercise is boosting Treg activity elsewhere in the body as well."