A factor has been derived from human leukocytes which irreversibly inhibits the response of human neutrophils to diverse chemotactic stimuli without impairing their viability. It is released by both polymorphonuclear and mononuclear leukocytes during incubation in acidic medium, after endotoxin exposure and subsequent incubation in low potassium medium, and during phagocytosis of particles. It is extractable from both leukocyte types and therefore must be preformed. This chemotactic inhibitor is completely separable from contaminating chemotactic activity in the crude supernatants, has a mol wt of 5000, and is inactivated by digestion with trypsin or chymotrypsin. It has been termed a neutrophil-immobilizing factor because it inhibits neutrophils directly and independently of the chemotactic stimulus, and has relatively little effect on human monocyte chemotaxis.

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