The binding properties of rabbit antibodies against various penicillins as studied by immunoprecipitation, indirect haemagglutination and radiometric immunoassay.

Rabbit antibodies raised against protein conjugates of azidocillin (AzO), ampicillin (AmpO), benzylpenicillin (BPO) and crude 6-aminopenicillanic acid (6-APA) gave rise to two precipitation lines in immunoprecipitation tests against penicillin protein conjugates where the carrier was unrelated to the immunogens. This pattern verifies extensive cross-reactions between penicillins, which have previously been reported. The use of AzO-coated rabbit red cells in indirect haemagglutination (IHA) inhibition tests showed good reactivity (a) between the inhibitor AzO and antibodies raised against AzO, AmpO and BPO, (b) between the inhibitor AmpO and antibodies raised against AmpO and BPO, (c) between the inhibitor BPO and antibodies raised against BPO. When BPO-coated red cells were used, AzO, AmpO and BPO inhibited the IHA activity of antibodies against the BPO. Varying IHA results were obtained using differently conjugated rabbit red cells. Thus antibodies against AmpO gave higher titres when using AzO- than BPO-coated red cells while for antibodies against BPO the opposite was noted. The radiometric assay utilizing AzO-serum albumin as tracer as well as reference substance resulted in significantly higher values for antisera against AzO and AmpO compared with those for antisera against crude 6-APA. Antisera against BPO were not analysed in this respect. Furthermore, there was poor correlation between the results obtained with the indirect IHA technique and the radiometric immunoassay.