A possible explanation for the discrepancy between ELISA and neutralising antibodies to tetanus toxin

The structure and protective activity of tetanus antibodies elicited in rab bits after whole-cell pertussis diphtheria-tetanus vaccine (DTPw) vaccinati on was studied. ELISA antibody levels and toxin neutralisation activity (TN T) were measured in individual serum samples. The ratio of symmetric and as ymmetric (functionally monovalent) Ige molecules was determined by concanav alin A (Con A) chromatography. This test is based on the fact that the carb ohydrate group responsible for the molecular asymmetry has high affinity fo r the lectin Con A, Asymmetric molecule ratio was observed to increase with immunisation time, as well as differences between TNT and ELISA levels. Al l serum samples were overestimated by ELISA as compared to TNT assay, in li ne with the markedly higher proportion of asymmetric molecules which have l ower toxin neutralising activity, Protective levels could not be predicted reasonably from ELISA results below 0.222 IU/ml, because this methodology f ails to discriminate between both types of antibodies and only an in vivo s erum neutralisation procedure (TNT) reflects the true neutralising serum ac tivity. (C) 2000 Elsevier Science Ltd. All rights reserved.