After exiting the circulation, leukocytes move in the tissues toward the site of injury by a process called chemotaxis, which is defined as locomotion along a chemical gradient.
Both exogenous and endogenous substances can act as chemoattractants.
Exogenous agents are bacterial products, including peptides that possess an N-formylmethionine terminal amino acid and some lipids.
Endogenous chemoattractants include several chemical mediators
(1) Cytokines, particularly those of the chemokine family
(2) Components of the complement system, particularly C5a
(3) Arachidonic acid (AA) metabolites, mainly leukotriene B4.
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