Platelet aggregation and release associated with immune complex formation in pig plasma.

The correlation between the formation of immune complexes and platelet aggregation and release was studied in pig plasma. The pigs were hyperimmunized with a multivalent antigen, horse ferritin. The ability of the antisera to form precipitating (insoluble) and soluble immune complexes was measured and correlated with the concomitant platelet aggregation and release of serotonin and LDH. Aggregation ws measured by aggregometry, differential centrifugation, and electron microscopy, and release by [14C]serotonin and LDH loss. Aggregation correlated with release (r = 0.84, p < 0.001), but no correlation was found between antibody titer or varying antigen concentrations and platelet [14C]serotonin release. A positive correlation was found between platelet [14C]serotonin release and precipitating immune complexes (r = 0.68, p < 0.001) and a negative correlation with soluble complexes (r = -0.78, p < 0.001). Maximal precipitation of insoluble complexes was complete within 30 sec or less, was independent of temperature, and obeyed the laws of mass action and of optimal ratios of antigen and antibody concentrations (equivalence zone). Platelet aggregation and serotonin release in plasma and precipitating immune complex formation were both maximal at antigen-antibody equivalence and undetectable at marked antigen or antibody excess. These results suggest that immune complex precipitation may initiate platelet aggregation and release in plasma and that the reaction may be at least partially blocked by soluble immune complexes.