Recruitment of leukocytes

At sites of infection, macrophages that have encountered microbes produce cytokines, such as Tumour Necrosis Factor (TNF) and Interleukin-1 (IL-1), that activate the endothelial cells of nearby venules to produce selectins, ligands for integrins, and chemokines.  Selectins mediate weak tethering and rolling of blood leukocytes, such as neutrophils on the endothelium; integrins mediate firm adhesion of neutrophils; and chemokines increase the affinity of neutrophil integrins and stimulate the migration of the cells through the endothelium to the site of infection.  Blood neutrophils, monocytes, and activated T lymphocytes use essentially the same mechanisms to migrate to sites of infection.