Influence of standards and antibodies in immunochemical assays for quantitation of immunoglobulin G in bovine milk.

This study reports the influence of using various reference antigens or standards as well as the source of antibody in the immunochemical quantitation of bovine immunoglobulin (Ig) G. Standard curves from analyses by ELISA and radial immunodiffusion were compared for bovine IgG from serum, colostrum, and cheese whey and for the two subclasses IgG1 and IgG2. Also compared were different sources of polyclonal antibodies (antisera from rabbit and sheep or hen yolk) and monoclonal antibodies that had various antibody specificities. The results indicate that IgG1 was a reliable alternative to purified IgG from milk or cheese whey for quantitation of IgG in milk based on ELISA absorbance. Serum IgG, colostral IgG, and IgG2 greatly underestimated milk IgG, regardless of the source of antibody used. Determination of milk IgG by radial immunodiffusion using antisera that were specific for rabbit anti-bovine IgG (H + L) was less dependent on the source of IgG that was used as the standard antigen. However, radial immunodiffusion using subclass-specific antibodies led to inaccurate estimation of IgG in milk unless the quantitation was based on a standard curve for IgG from milk. The Ig from hen yolk were a feasible alternative source of specific antibodies for immunoassay of IgG in bovine milk.