During anaphylaxis in the rabbit, whole blood serotonin and histamine decrease, plasma serotonin and histamine increase, and the lung content for these amines is transiently elevated. The drop in whole blood serotonin and histamine is due to the disappearance of the platelets which are caught in the lung. Injection of the antigen into the systemic circulation produced the same results, indicating the phenomenon to be specific mainly for the lung. The in vitro results with normal blood and the in vivo effects in normal rabbits caused by the antigen-antibody complex were the same as those seen when a sensitized rabbit and the specific antigen were used. Speculation as to the role of platelet serotonin in human hypersensitivity reaction is made. The presence of histamine and the lack of serotonin in human lung as well as the relationship to human allergic pulmonary manifestations, are discussed. The pulmonary aspects of platelet thrombi in fatal anaphylaxis in rabbits are considered.
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