Migration (Leukocyte Extravasation)
In the collection of Dr. Rodger McEver, Vice President of Research, Oklahoma Medical Research Foundation.
11" h x 18" w
How do white blood cells find their targets. These blood cells protect us against infection, but infections can occur anywhere in the body. In spite of their name, white blood cells aren't limited to the blood? How do they get out, and how do they do this in the right place - the site of infection?
In this piece, two leukocytes (white blood cells) migrate from blood, squeezing through openings in the thin wall of a blood vessel, into tissue where they engulf and destroy bacteria such as Pseudomonas. The leukocytes use a chemical roadmap: they can "smell" chemicals released at the site of infections (and some tumors.)
Leukocytes are attracted by chemokines -– “cellular perfumes” released by our body in response to invading bacteria or viruses, and also by chemicals produced by the microbes themselves. They essentially "smell" the infection using a wide variety of receptor proteins which have evolved to recognize pathogenic microorganisms to which humans have been exposed throughout evolution.
The movement of leukocytes out of the blood and into tissue, known as "extravasation," is mediated by a series of adhesion molecules on both the activated leukocytes and the blood vessel cells. These adhesion molecules appear or are activated by tissue damage (caused by infection) or by chemicals released by the microbes themselves. Like Velcro, the adhesion molecules on the blood vessel walls and those on the leukocytes match and lead first to slowing down and adhering of leukocytes to the vessel walls, followed by extravasation from inside the blood vessel to out into the tissues. There, the leukocytes can fight infection. Sometimes, this process can contribute to inflammation. Tissue damage associated with inflammation also triggers extravasation, and the migrating leukocytes may exacerbate the tissue damage. Drugs have been developed which successfully block extravasation of leukocytes by blocking the adhesion process. One example is Biogen's Tysabri, for Crohn's disease and multiple sclerosis.