The heat-stable antibacterial activity of rabbit serum against Gram-positive microorganisms has been shown to reside in a cationic protein fraction of platelet lysosomal granules. The activity is released during platelet aggregation. No plasma or serum component is required for the bactericidal effect. The platelet bactericidin resembles the antibacterial proteins of leukocyte granules both in cellular localization and in biochemical characteristics. It can be differentiated from platelet factor 4, the antiheparin factor, which is also a basic protein in platelet granules. The antibacterial effect of the platelet bactericidin may be related to the metabolic activity of the organisms. This antibacterial activity of platelets may represent another means by which platelets can participate in host inflammatory defense reactions.

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