Editing Immune system (section)

====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>