Regulated intravenous doses of quinidine were given to patients with the antibody of quinidine purpura to produce controlled thrombocytopenia without clinical sequelae. The degree of thrombocytopenia and the rate at which it developed were dependent on the relative plasma concentration of quinidine and antibody. By relating in vivo changes in platelet levels to concurrent in vitro tests for antibody activity and to quantitative relationships between reactants determined in Papers I and III of this series, it was concluded that the amount of antibody which attaches to platelets when thrombocytopenia develops is insufficient to cause complement fixation or platelet agglutination. Platelets do not appear to be destroyed directly by reaction with antibody in vivo. The minimal amount of antibody which does attach to platelets in vivo appears to increase their susceptibility to the usual mechanisms of sequestration.

Megakaryocytes and blood vessels do not appear to be affected directly by the antibody which causes quinidine purpura, and hemorrhagic manifestations of the disease appear to be consequent to changes in platelets alone.

A safe method of performing in vivo tests for the presence of an antibody of drug purpura is described.

The implications of the present work in idiopathic thrombocytopenic purpura are discussed.

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