Responses of human platelets to immunologic stimuli: independent roles for complement and IgG in zymosan activation.
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.
D000361:Agammaglobulinemia / Q000097:blood; D001792:Blood Platelets / Q000502:physiology*; D003165:Complement System Proteins; D005340:Fibrinogen; D006801:Humans; D007141:Immunoglobulin Fc Fragments; D007074:Immunoglobulin G; D010974:Platelet Aggregation; D012701:Serotonin / Q000378:metabolism; D015054:Zymosan / Q000276:immunology*
D007141:Immunoglobulin Fc Fragments; D007074:Immunoglobulin G; D012701:Serotonin; D005340:Fibrinogen; D003165:Complement System Proteins; D015054:Zymosan