Platelet aggregation by isolated and aggregated human IgG.

Isolated myeloma proteins of the four human IgG subclasses, aggregated by heat of bis-diazotized benzidine (BDB), induced aggregation of human platelets (Pl.A.) demonstrable by the sedimentation pattern test. With this sensitive technique, approximately 0-1 mug/ml of BDB-aggregated IgG1 and IgG3 could be detected whereas 200 to 1000 times higher concentrations of IgG2 and IgG4 were required for the Pl.A. reaction. The reaction pattern showed similarities with complement fixation tests.