Editing Immune system (section)

===Physical exercise===
Physical exercise has a positive effect on the immune system and depending on the frequency and intensity, the pathogenic effects of diseases caused by bacteria and viruses are moderated.<ref name="pmid32728975">{{cite journal |vauthors=da Silveira MP, da Silva Fagundes KK, Bizuti MR, Starck É, Rossi RC, de Resende E, Silva DT |title=Physical exercise as a tool to help the immune system against COVID-19: an integrative review of the current literature |journal=Clinical and Experimental Medicine |volume=21 |issue=1 |pages=15–28 |date=February 2021 |pmid=32728975 |pmc=7387807 |doi=10.1007/s10238-020-00650-3}}</ref> Immediately after intense exercise there is a transient immunodepression, where the number of circulating lymphocytes decreases and antibody production declines. This may give rise to a window of opportunity for infection and reactivation of latent virus infections,<ref name="pmid27909225">{{cite journal |vauthors=Peake JM, Neubauer O, Walsh NP, Simpson RJ |title=Recovery of the immune system after exercise |journal=Journal of Applied Physiology |volume=122 |issue=5 |pages=1077–1087 |date=May 2017 |pmid=27909225 |doi=10.1152/japplphysiol.00622.2016|s2cid=3521624 |url=https://researchonline.ljmu.ac.uk/id/eprint/16304/3/Recovery%20of%20the%20immune%20system%20after%20exercise.pdf }}</ref> but the evidence is inconclusive.<ref name="pmid29713319">{{cite journal |vauthors=Campbell JP, Turner JE |title=Debunking the Myth of Exercise-Induced Immune Suppression: Redefining the Impact of Exercise on Immunological Health Across the Lifespan |journal=Frontiers in Immunology |volume=9 |issue= |pages=648 |date=2018 |pmid=29713319 |pmc=5911985 |doi=10.3389/fimmu.2018.00648|doi-access=free }}</ref><ref name="pmid32139352">{{cite journal |vauthors=Simpson RJ, Campbell JP, Gleeson M, Krüger K, Nieman DC, Pyne DB, Turner JE, Walsh NP |title=Can exercise affect immune function to increase susceptibility to infection? |journal=Exercise Immunology Review |volume=26 |issue= |pages=8–22 |date=2020 |pmid=32139352 |doi=}}</ref>

====Changes at the cellular level ====
[[File:Neutrophils.jpg|right|thumb|Four neutrophils in a [[Romanowsky stain|Giemsa-stained]] blood film]]

During exercise there is an increase in circulating [[leukocytes|white blood cells]] of all types. This is caused by the frictional force of blood flowing on the [[endothelial cell]] surface and [[catecholamine]]s affecting [[β-adrenergic receptor]]s (βARs).<ref name="pmid27909225"/> The number of [[neutrophils]] in the blood increases and remains raised for up to six hours and [[Left shift (medicine)|immature forms]] are present. Although the increase in neutrophils ("[[neutrophilia]]") is similar to that seen during bacterial infections, after exercise the cell population returns to normal by around 24 hours.<ref name="pmid27909225"/>

The number of circulating [[lymphocyte]]s (mainly [[natural killer cells]]) decreases during intense exercise but returns to normal after 4 to 6 hours. Although up to 2% of the cells [[apoptosis|die]] most migrate from the blood to the tissues, mainly the intestines and lungs, where [[pathogen]]s are most likely to be encountered.<ref name="pmid27909225"/>

Some [[monocyte]]s leave the blood circulation and migrate to the muscles where they differentiate and become [[macrophage]]s.<ref name="pmid27909225"/> These cells differentiate into two types: proliferative macrophages, which are responsible for increasing the number of [[Myogenesis|stem cell]]s and restorative macrophages, which are involved their maturing to muscle cells.<ref name="pmid34786967">{{cite journal |vauthors=Minari AL, Thomatieli-Santos RV |title=From skeletal muscle damage and regeneration to the hypertrophy induced by exercise: what is the role of different macrophage subsets? |journal=American Journal of Physiology. Regulatory, Integrative and Comparative Physiology |volume=322 |issue=1 |pages=R41–R54 |date=January 2022 |pmid=34786967 |doi=10.1152/ajpregu.00038.2021|s2cid=244369441 }}</ref>