Leukocyte Recruitment to Sites of Inflammation

Leukocytes are recruited from the blood into the extravascular
tissue, where infectious pathogens or damaged
tissues may be located, and are activated to perform their
functions.
• Leukocyte recruitment is a multi-step process consisting
of loose attachment to and rolling on endothelium (mediated
by selectins); firm attachment to endothelium (mediated
by integrins); and migration through interendothelial
spaces.
• Various cytokines promote expression of selectins and
integrin ligands on endothelium (TNF, IL-1), increase the
avidity of integrins for their ligands (chemokines), and
promote directional migration of leukocytes (also chemokines);
many of these cytokines are produced by tissue
macrophages and other cells responding to pathogens or
damaged tissues.
• Neutrophils predominate in the early inflammatory infiltrate
and are later replaced by macrophages.