A10-year-old male child complains of pain and swelling in the left foot. He gives a history of injury while playing. Migration of leukocytes to the site of injury is mediated by

LEUKOCYTE RECRUITMENT • The journey of leukocytes from the vessel lumen to the tissue is a multistep process that is mediated and controlled by adhesion molecules and cytokines called chemokines LEUKOCYTE ADHESION • Extravasation is the sequence of events in the journey of leukocytes from vessel lumen to interstitial tissue which include the following steps > Margination - leukocytes assume a peripheral portion of lumen along endothelial surface > Rolling - leukocytes adhere transiently, detach and bind again and finally adhere firmly > Rolling is mediated by Selectins (P, L and E-selectins) > Firm adhesion is mediated by Integrins (LFA-1. MAC-1, VLA-4, α4β7) > Pavement - endothelium virtually lined by leukocytes LEUKOCYTE MIGRATION • Diapedesis or transmigration > Leukocyte migration through endothelium > Occurs mainly in post capillary venules > Adhesion molecules involved - CD31 or PECAM-1 (platelet endothelial cell adhesion molecule) CHEMOTAXIS • Leukocyte migration toward the site of injury • Locomotion oriented along a chemical gradient • Chemo-attractants > Exogenous - bacterial products (most common) > Endogenous ■ Cytokines, particularly - IL-8 ■ Complement, particularly - C5a ■ Arachidonic acid metabolites particularly - LTB4 • Neutrophils predominate in the inflammatory infiltrate during the first 6 to 24 hours • Neutrophils are replaced by monocytes in 24 to 48 hours • In Pseudomonas infection - cellular infiltrate is dominated by neutrophils for several days • In viral infections, lymphocytes may be the first cells to arrive