Human platelets aggregate during antigen-antibody reactions at different antigen-antibody ratios

The effects of immune complex formation on human platelets were tested in vitro using tetanus-toxoid and rabbit anti-tetanus toxoid for immune complex formation and washed human platelets. The antigen-antibody reactions took place at 37 degrees C in a stirred cuvette, constantly monitored. To this cuvette we added initially a platelet suspension and a dilution of antibody, and after this mixture reached equilibrium, antigen was added. Platelets aggregated as a consequence of immune reactions taking place at antigen-antibody ratios ranging from four times antigen excess to twenty times antibody excess (relative to equivalence). Platelets also aggregated as a result of reactions in which heat-inactivated antiserum was used, although in a more narrow range of Ag:Ab concentrations, suggesting that complement is not essential for their stimulation but may help in stabilizing the Ag:Ab reaction. Our observations provide indirect evidence in support of the involvement of human platelets in the pathogenesis of immune complex-mediated hypersensitivity reactions.