The immunological release of vasoactive amines from rabbit platelets by a mechanism requiring blood leukocytes has been described. The reaction involved leukocytes from an immunized rabbit, antigen, and platelets and did not require the complement system. The leukocytes appeared to be mononuclear, had the ability to adhere to glass, and were found in greater numbers in the blood than in spleen, lymph nodes, thoracic duct lymph, bone marrow, and peritoneal cavity washings.

Part or all of the effect on the platelets appeared to result from active release of a soluble factor from the leukocytes, which induced vasoactive amine release from the platelets. The platelets were not lysed during this reaction and the release of vasoactive amines required the active participation of metabolic pathways in the platelet.

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