Nieman published a paper in Immunology and Cell Biology that explained how exercise affects the human immune system. The open window theory says that the immune system is weakened for 3-72 hours following extended, strenuous exercise. Monocytes and neutrophils invade the inflamed area that results from muscles injured during exercise. However, the ability of the neutrophils to phagocyte foreign invaders has been shown to be limited (Müns 1993). Salivary excretion is reduced up to 18 hours after intense exercise (Steerenberg 1997). A 1997 study by Neiman found that natural killer cell activity is reduced after exercise. Cytokine concentration is altered by intense exercise. Exercise resulting in muscle soreness is linked to a stronger anti-inflammatory response.