An assay to measure antigen-specific immune complexes in food-allergy patients

An allergen- and isotype-specific assay to quantify the presence of serum immune complexes is described. This assay is based on a two-site recognition system so that only molecular complexes containing both antigen and antibody are counted. In this study affinity-purified antibodies to egg and milk whey were used on paper discs in conjunction with a soluble, labeled anti-immunoglobulin (IgE or IgG specific). To evaluate the assay, immune complexes were prepared in vitro by mixing antigen at different concentrations with serum from a patient with high levels of antibody. Complexes were detectable over a 5 log range of antigen concentration. The lower limit of sensitivity of the assay was estimated to be 5.8 ng of complexed IgE per milliliter of patient serum. To determine if the assay could detect natural in vivo-formed complexes, sera from patients with positive RAST scores to egg white or milk were selected for a preliminary study. Three of the nine egg white-positive sera and two of the nine milk-positive sera were found to contain significant levels of IgE immune complexes. The versatility and sensitivity of the assay should now make it feasible to design experiments to elucidate the role, if any, of circulating immune complexes in the etiology of food allergy.