Leukocyte recruitment has been recognized as an early event in inflammatory processes since the late 19th century. Accumulation and trafficking of leukocytes in tissues under physiological and pathological conditions are orderly (typically neutrophils precede mononuclear cells) and selective because, in certain states, one or more leukocyte subsets are recruited preferentially (e.g., eosinophils in allergy). The current paradigm of recruitment is that of a multistep process involving the action of chemotactic signals (1 ,2 ).