is the process by which phagocytic cells enter the tissues from capillaries
Phagocyte To reach the site of infection, phagocytes leave the bloodstream and enter the affected tissues. Signals from the infection cause the endothelial cells that line the blood vessels to make a protein called selectin, which neutrophils stick to on passing by. Other signals called vasodilators loosen the junctions connecting endothelial cells, allowing the phagocytes to pass through the wall. Chemotaxis is the process by which phagocytes follow the cytokine "scent" to the infected spot.[2] Neutrophils travel across epithelial cell-lined organs to sites of infection, and although this is an important component of fighting infection, the migration itself can result in disease-like symptoms.[64] During an infection, millions of neutrophils are recruited from the blood, but they die after a few days.[65]
Interstitial fluid Not all of the contents of the blood pass into the tissue fluid, which means that tissue fluid and blood are not the same. Red blood cells and platelets cannot pass through the walls of the capillaries. The resulting mixture that does pass through is, in essence, blood plasma with lower concentration of plasma proteins. Tissue fluid also contains some types of white blood cell, which help combat infection.