During states of inflammation, white blood cells (leukocytes) play a key role in maintaining tissue homeostasis through elimination of pathogens and removal of damaged tissue. Leukocytes migrate to the site of inflammation by crawling over and through the blood vessel wall, into the tissue. This term refers to a defect in the attachment of leukocytes to the blood vessel wall, which is a key step required before they can pass through gaps of the endothelial cells of the blood vessel wall to migrate to the site of inflammation. [from
HPO]