The consensus among exercise immunologists is that acute exercise may cause alterations in the immune system that can persist for several hours and/or days. Significant immune suppression and/or upper airway inflammation has been reported during recovery from prolonged or strenuous exercise. The former is believed to increase the risk of upper respiratory illness, lending to the recovery period being called the “open window.” Nieman's “J-curve” model suggests a strong link between intensity and duration of exercise training and the risk of upper respiratory tract infection (URTI). Given the importance of the innate immune system as a first line of defense against infection, natural killer (NK) cells have been studied extensively and NK-cell number and activity frequently measured as an index of host defense following exercise.

The ...