^ Colbert LH, Visser M, Simonsick EM, Tracy RP, Newman AB, Kritchevsky SB, Pahor M, Taaffe DR, Brach J, Rubin S, Harris TB (July 2004). "Physical activity, exercise, and inflammatory markers in older adults: findings from the Health, Aging and Body Composition Study". Journal of the American Geriatrics Society. 52 (7): 1098–104. doi:10.1111/j.1532-5415.2004.52307.x. PMID 15209647.
Physical activity boosts the immune system. This is dependent on the concentration of endogenous factors (such as sex hormones, metabolic hormones and growth hormones), body temperature, blood flow, hydration status and body position. Physical activity has shown to increase the levels of natural killer (NK) cells, NK T cells, macrophages, neutrophils and eosinophils, complements, cytokines, antibodies and T cytotoxic cells. However, the mechanism linking physical activity to immune system is not fully understood.