A New Method for the Simultaneous Quantitation of Platelet-bound Immunoglobuin (IgG) and Complement (C3) Employing an Enzyme-linked Immunosorbent Assay (ELISA) Procedure

An ELISA technique for the quantitation of platelet-bound IgG and C3 is described. This is an antiglobulin consumption assay using commercial horseradish peroxidase conjugated anti-IgG and anti-C3 antisera. The greater the amount of antiglobulin consumed by the reaction with platelets the less is available to bind to an immune adsorbent in the form of human serum-coated polystyrene balls. This test can be calibrated by adding known quantities of IgG of C3 to the specific enzyme-linked antibody and the unbound fraction of antiserum is quantitated by allowing it to react with a chromogenic substrate for the enzyme and measuring the intensity spectrophotometrically, thus establishing an inverse relationship with the amount of antigen. Platelets from normal donors and those with non-immunological thrombocytopenia gave values of 1.3--15.5 ng IgG/10(6) platelets and 0.22--0.96 ng C3/10(6) platelets which are in accordance with the normal ratio of these proteins in normal serum. Fifteen patients in whom immune destruction of platelets weas suspected had excess platelet-bound IgG ranging from 30 to 450 ng IgG/10(6) platelets. In none of these patients could excess platelet-bound C3 be demonstrated. Compared to the antiglobulin consumption test we have found this test to be superior both technically and in terms of sensitivity and reproducibility.