Simultaneous quantification of red cell and platelet surface-bound IgG & IgM by an ELISA technique

In vivo red cell-bound (RBC-) and platelet-bound (P-) IgG and IgM were measured by an enzyme-linked immunosorbent assay (ELISA) using washed cells as solid phase and alkaline phosphatase conjugated antiglobulins (Fc-specific). With this technique cells from normal donors had small amounts of RBC-IgG ranging from 0.02 to 0.41 A405 (absorbance at 405 nm wavelength) (10(8) h)-1, of RBC-IgM ranging from 0.01 to 0.13 (10(8)h)-1, of P-IgG ranging from 0.00 to 0.16 (10(7)h)-1 and of P-IgM ranging from 0.00 to 0.05 (10(7)h)-1. 13 of 14 patients with positive RBC direct antiglobulin test had increased RBC IgG and/or RBC IgM (P less than 0.01). 14 of 16 patients with idiopathic autoimmune thrombocytopenic purpura had increased P-IgG and/or P-IgM (P less than 0.01). 10 of 13 patients with circulating immune complexes had increased RBC-IgG and/or IgM (P less than 0.01) and 12 of 13 patients increased P-IgG and/or IgM (P less than 0.01). The direct ELISA is useful for quantification of in vivo surface-bound RBC-Ig and P-Ig autoantibodies and receptor-bound immune complex-associated Ig and requires only standard laboratory equipment.