<P> In the blood, neutrophils are inactive but are swept along at high speed . When they receive signals from macrophages at the sites of inflammation, they slow down and leave the blood . In the tissues, they are activated by cytokines and arrive at the battle scene ready to kill . </P> <P> When an infection occurs, a chemical "SOS" signal is given off to attract phagocytes to the site . These chemical signals may include proteins from invading bacteria, clotting system peptides, complement products, and cytokines that have been given off by macrophages located in the tissue near the infection site . Another group of chemical attractants are cytokines that recruit neutrophils and monocytes from the blood . </P> <P> 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 . 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 . During an infection, millions of neutrophils are recruited from the blood, but they die after a few days . </P> <P> Monocytes develop in the bone marrow and reach maturity in the blood . Mature monocytes have large, smooth, lobed nuclei and abundant cytoplasm that contains granules . Monocytes ingest foreign or dangerous substances and present antigens to other cells of the immune system . Monocytes form two groups: a circulating group and a marginal group that remain in other tissues (approximately 70% are in the marginal group). Most monocytes leave the blood stream after 20--40 hours to travel to tissues and organs and in doing so transform into macrophages or dendritic cells depending on the signals they receive . There are about 500 million monocytes in one litre of human blood . </P>

Is the process by which phagocytic cells enter the tissues from capillaries