Relative functional binding activity of IgG1 and IgG3 anti-D in IgG preparations

The intention to replace polyclonal IgG anti-D with human monoclonal antibody in the prophylaxis of haemolytic disease of the newborn requires knowledge concerning the relative content of IgG1 and IgG3 anti-D in prophylactic IgG preparations that are in present use. This has been carried out using a functional assay in which the absolute amount of IgG1 and IgG3 anti-D present on red cells was determined after incubation with IgG preparations. The assay was carried out by flow cytometry on 17 samples; expressed as a percentage of the total, the average value for the amount of IgG3 anti-D on the cells was 8% (range 1-18%). Similar measurements were also made on the anti-D present in 18 samples of antisera; IgG3 anti-D formed a larger fraction of the total, the average value being 17% (range 0-60%) confirming previously reported estimates. It is suggested that some of the low values found for IgG3 in IgG preparations may be due do preferential loss during production.