Radio‐Immunoassay of the Functional Activity of Anti‐D (Rh) Preparations using a Human Monoclonal ¹²⁵I‐Labelled Anti‐D

The prophylactic effect of anti-D in the prevention of maternal immunization to the D antigen of the Rh system depends in the first instance on its ability to bind to fetal Rh-positive red cells. The functional activity of the injected anti-D in this respect is dependent on both the plasma concentration and functional affinity constant of the antibody, and both factors should be taken into account in assessing IgG anti-D preparations for clinical use. A radio-immunoassay utilizing 125-I-labelled human monoclonal anti-D has been developed which measures the amount of antibody bound to red cells after the addition of an aliquot of the test anti-D preparation. The assay is thus a functional test in that it measures the ability of the anti-D to react with the antigen. The functional activity of the test anti-D is compared to that of the international standard, and the concentration is then expressed in terms of microgram-equivalents, that is, its equivalence in functional terms to the stated amount of international standard. The radio-immunoassay was compared to the currently used agglutination assay, and the results were found to give good agreement in 18 out of the 21 samples assayed.