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Human platelets are known to have Fc receptors that are able to recognize soluble immune complexes and to respond to that stimulation by aggregating and releasing soluble factors. In diabetes, enhanced platelet aggregation has been proposed as one of the factors contributing to the development of microangiopathy. Soluble immune complexes isolated from seven diabetic patients were found to enhance ADP-induced platelet aggregation and release of ATP. This enhancement was proven not to be an artifact due to the isolation protocol, by comparison of purified immune complexes with nonspecific protein purified from normal sera by identical or slightly modified isolation protocols. Soluble immune complex appear to be the first well-characterized platelet aggregating factors form the sera of diabetic patients. The natural of the antigen involved in their formation does not appear relevant, since very similar results were obtained whether soluble immune complexes were purified from patients with insulin-anti-insulin complexes in their serum, or from those without such complexed but with positive results in nonspecific screening techniques.

A simplified approach for estimating free and total anti-insulin antibodies and soluble insulin-anti-insulin immune complexes in serum has been developed and evaluated. For determination of free anti-insulin antibodies, a binding ratio for 125I-labeled insulin at fivefold serum dilution is calculated; the binding ratios obtained are reproducible (run-to-run coefficient of variation, 7.3%) and correlate well with titration of anti-insulin antibodies by other techniques (correlation coefficient, 0.9327). Total anti-insulin antibody concentrations are determined by calculating the [125I]insulin binding ratio for a sample previously acidified, adsorbed with acid dextran-coated charcoal, and neutralized. The difference in binding ratios between two aliquots of the same serum, one diluted 10-fold without manipulation and the second studied at an identical dilution after acidification and removal of free insulin, is taken as an index of the presence of soluble insulin-anti-insulin immune complexes. Because the tests can be performed at a single dilution, both time and materials are conserved without apparent loss of discrimination between sera with high antibody titers and high concentrations of insulin-anti-insulin immune complexes and sera with negative titers or low concentrations.