Prediction of the Severity of Haemolytic Disease of the Newborn: Quantitative IgG Anti‐D Subclass Determinations Explain the Correlation with Functional Assay Results

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Abstract
Sera containing anti‐D, taken from 44 RhD‐negative women with RhD‐positive infants, were tested in antibody‐dependent cellular cytotoxicity (ADCC) and monocyte monolayer assays (MMA) which used similar target and effector cell populations. In addition, the anti‐D concentration was measured in the Auto Analyzer and the number of IgG1 and IgG3 anti‐D molecules bound to the target red cells was measured by flow cytometry. The results of the functional assays and AutoAnalyzer quantitation were examined for correlation with IgG subclass quantitation and all results were compared for their ability to predict the severity of haemolytic disease of the newborn (HDN). ADCC correctly predicted HDN in 39/44 (88.6%) cases, AutoAnalyzer quantitation in 35/44 (79.5%) and the MMA in 32/44 (72.7%). For all three assays, the number of correct predictions was highest when the maternal serum contained both IgG1 and IgG3 anti‐D. ADCC activity and HDN were correlated with the number of cell‐bound IgG1 molecules (r≥0.58), but MMA activity was most closely correlated with the number of cell‐bound IgG3 molecules (r = 0.68). Hence the superior predictive value of ADCC is due to its ability to reflect the IgG1 component of maternal anti‐D, which has a better correlation than IgG3 anti‐D with the severity of HDN.