Serum IgE levels can be measured by reverse passive antiglobulin haemagglutination (RPAH) of trypsin-treated human or sheep red cells coupled to sheep IgG anti-human IgE by chromic chloride. The results show a high correlation with those obtained by the radioactive single radial immunodiffusion method. Interfering anti-sheep IgG factors can be easily removed by absorption with small amounts of whole sheep or bovine serum cross-linked with glutaraldehyde. Standardisation with the British standard for IgE shows that the detection limit of the RPAH method is 0.5 IU/ml (1.2 ng/ml). The system is therefore comparable in sensitivity to the paper radio-immunosorbent test, and has the advantages of being simple, rapid and cheap. The RPAH method can be used to measure any class of immunoglobulin. For IgA the detection limit is found to be 10––/sup>4 IU/ml (1.4ng/
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