Analysis of the zymosan (Z)-induced human platelet response with washed platelets, fresh agammaglobulinemic plasma, and purified human IgG has identified a previously unrecognized role of IgG in this reaction, in addition to the previously reported requirements for C and fibrinogen. The rate-limiting step involves the assembly of C components on the Z particles and is independent of immunoglobulin. After the heat-labile components (supplied by agammaglobulinemic plasma) are Z bound, Z-specific IgG (apparently antibody) can bind to Z very rapidly, conferring on the previously inactive Z-complement-fibrinogen intermediate the capacity to induce platelet aggregation and serotonin release. The critical function of this Z-bound IgG appears to be in the interaction of the active Z complex with the platelet through Fc receptors, since soluble Fc fragments of each IgG subclass can totally block platelet activation. It is proposed that the human platelet requires a dual, or multi-site, stimulation to become activated during some forms of immunologic injury. These findings may provide some insight into clinical syndromes involving thrombocytopenia and intravascular coagulation during sepsis.

1

This work was supported in part, by Research Grant A1-12568 and Training Grant 5T32HL07152-02 from the United States Public Health Service, and by a grant from the Monroe County Chapter of the Arthritis Foundation.

2

Presented at the Annual Meeting of the American Federation for Clinical Research, Washington, D.C., May 2, 1977 (Clin. Res. 25:363A, 1977).

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