Influence of antibody valency in a displacement immunoassay for the quantitation of 2,4-dichlorophenoxyacetic acid

The influence of antibody valency in a displacement immunoassay was investi gated by comparing the whole antibody molecule with the corresponding Fab-f ragment. The displacement immunoassay for the herbicide 2,4-dichlorophenoxy acetic acid (2,4-D) takes advantage of the cross-reactivity of monoclonal a nti-2,4-D antibodies and the Fab-fragments toward immobilized 2-methyl-4-ch lorophenoxyacetic acid (MCPA). Due to the low affinity of the antibodies to ward MCPA (cross-reactivity of approximately 30%), the addition of 2,4-D re sulted in a displacement of the antibodies or the fragments. The detection limits obtained with whole anti-2,4-D antibodies and Fab-fragments were 0.1 mu g/l and 0.01 mu g/l 2,4-D, respectively. The whole antibodies and the F ab-fragments show similarities, such as the cross-reactivity toward MCPA (2 6% and 33%), and some characteristics of the calibration curve, for example the large detection range and the sensitivity. In contrast to the bivalent antibodies, however, increasing the hapten/protein ratios of the immobiliz ed MCPA-BSA conjugates did not affect the detection limit when using the Fa b-fragments. Moreover, kinetic experiments reveal a faster displacement rea ction with the Fab-fragments. A disadvantage of using the Fab-fragments is the generation of lower absorbance values in the ELISA. (C) 1999 Elsevier S cience B.V. All rights reserved.