Main feature of chemotaxis is

Ref Robbins 8/e p45 ..9/e p77 Chemotaxis. After extravasating from the blood, leuko- cytes move toward sites of infection or injury along a chem- ical gradient by a process called chemotaxis. Both exogenous and endogenous substances can be chemotactic for leuko- cytes, including the following: * Bacterial products, paicularly peptides with N-formyl- methionine termini * Cytokines, especially those of the chemokine family * Components of the complement system, paicularly C5 * Products of the lipoxygenase pathway of arachidonic acid (AA) metabolism, paicularly leukotriene B4 (LTB4) These mediators, which are described in more detail later, are produced in response to infections and tissue damage and during immunologic reactions. Leukocyte infiltration in all of these situations results from the actions of various combinations of mediators. Chemotactic molecules bind to specific cell surface receptors, which triggers the assembly of cytoskeletal con- tractile elements necessary for movement. Leukocytes move by extending pseudopods that anchor to the ECM and then pull the cell in the direction of the extension. The direction of such movement is specified by a higher density of chemokine receptors at the leading edge of the cell. Thus, leukocytes move to and are retained at the site where they are needed. The type of emigrating leukocyte varies with the age of the inflammatory response and with the type of stimulus. In most forms of acute inflammation, neutrophils predomi- nate in the inflammatory infiltrate during the first 6 to 24 hours and are replaced by monocytes in 24 to 48 hours (Fig. 2-6). Several factors account for this early abundance of neutro- phils: These cells are the most numerous leukocytes in the blood, they respond more rapidly to chemokines, and they may attach more firmly to the adhesion molecules that are rapidly induced on endothelial cells, such as P- and E-selectins. In addition, after entering tissues, neutrophils are sho-lived--they die by apoptosis and disappear within 24 to 48 hours--while monocytes survive longer. There are exceptions to this pattern of cellular infiltration,