The alveolar macrophage was studied in parabiotic rats using an inbred strain. Parabiotic pairs were sutured together at five weeks of age. Rats were subjected to a full thickness cutaneous burn of 20 per cent of the body surface area at seven weeks of age, and alveolar macrophages were washed from the lungs at six days post burn. The number of alveolar macrophages, their per cent of activation, and their ability to phagocytize and kill P aeruginosa in vitro were significantly increased at six days post burn in the burned controls and in both the burned and unburned members of the parabiotic pairs. No change in the alveolar macrophages was found in either unburned parabiotic pairs or in those which were sham-burned. These results indicate that a humoral or cellular agent produced either within the cutaneous burn wound or elsewhere as a response to the injury, traverses the parabiotic cross circulation to stimulate the alveolar macrophages.
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