The Innate Immune System

P selectin binds neutrophil weakly so that it 'rolls' along the endothelial surface.

In order to reach the site of infection, the neutrophil has to be able to leave the bloodstream and enter the affected tissue. The process by which a leukocyte migrates and moves through the endothelium into a site of tissue infection or inflammation is called diapedesis, or leukocyte extravasation. In response to inflammation, specific receptors on the neutrophil cell membrane binds onto selectin receptors on the surface of the endothelium. This slows down the progress of the neutrophil within the blood stream and causes it to ‘roll’ against the inner surface of the blood vessel wall. Chemokines on the endothelium of the inflamed tissue enable neutrophils that are weakly bound to the endothelium to express integrins on their cell membrane. Integrins in turn bind to CAMs such as ICAM-1 receptors on the endothelial cells and this causes a tighter bond between the neutrophil and endothelium. Vasodilators such as histamine and bradykinin loosen the junctions between the neighbouring endothelial cells. This enables the bound neutrophils to ‘crawl’ along the endothelial surface, until they can squeeze through a gap junction between two endothelial cells and passage out of the bloodstream into the inflamed tissues.