Which statement BEST summarizes the association between exercise and improved immunity?
A) Long-term, heavy exercisers get the most gain in immunity versus those who exercise only moderately.
B) Moderate-intensity exercise temporarily increases the number of white blood cells, thereby boosting immunity.
C) Regular exercise does not have any correlation to a healthy immune system.
D) The most significant improvements in immunity are seen when a moderate exerciser begins a more intense exercise program.