Regular wrestling could boost children's immune systems, a study suggests.
Wrestling said to improve health
University of California researchers found the number of bacteria and virus fighting white blood cells was significantly raised by exercise.
They said in the British Journal of Sports Medicine that natural killer cells rose in particular.
However, sports science experts said white blood cell counts alone were not evidence of improved immunity.
A separate study, published in the same journal, highlighted the benefits of exercise in all age groups, finding that martial arts promote fitness in middle age.
The research into children put 11 healthy boys aged between 14 and 18 through a 1.5-hour wrestling practice session and sampled their blood before and after the session.
They found that levels of all groups of white blood cells, particularly the natural killer cells, rose significantly.
The killer cells fight cancer and viruses by searching out the equivalent of a flag carried by normal cells. Those without such a flag are attacked by a cocktail of chemicals.
The researchers said this was the first time it had been shown in healthy adolescents that exercise had a significant effect on the immune system. But it was not yet possible to say what the overall effect was on immunity.
However, Professor Mike Gleeson, professor of sports medicine at Loughborough University, said: "There is a problem in interpreting increases in numbers of white blood cells as a result of exercise as improving the immune system and protecting against infection."
What the white blood cells were doing was more important than how many of them there were, he said. Counting the increase in specific types of white blood cell was also not proof the exercise was beneficial, Professor Gleeson added.
He said regular, moderate amounts of exercise of no more than two hours a day probably did have a slight benefit on the immune system.
The martial arts study compared fitness levels of nine people aged between 40 and 60 who practised soo bahk do, which is similar to karate, with nine sedentary people.
Those who did the martial art were found to have better levels of body fat, balance, flexibility, muscle strength, endurance and aerobic capacity. There was no benefit for grip strength.