Affinity of IgE and IgG against cross-reactive carbohydrate determinants on plant and insect glycoproteins

Background: Cross-reactive carbohydrate determinants (CCDs) are probably the most widely occurring IgE epitopes. Approximately one fifth of patients with allergy develop IgE antibodies against such glycans. However, they appear to be of low clinical significance. Objective: We wanted to elucidate the reasons for this lack of clinical symptoms on contact with CCD allergens by determination of the binding affinities of patients' IgE and IgG antibodies. Methods: IgE and IgG against CCDs were affinity-purified from sera of selected patients. The binding affinity to defined glyco-epitopes was measured by surface plasmon resonance. Results: From a pool of CCD-positive sera, we isolated 0.1 and 25 mu g CCD-specific IgE and IgG, respectively. The binding affinity of purified IgE antibodies to core alpha 1,3-fucosylated glycans was in the 10(-10) mol/L range. The affinity was highest when both fucose and xylose were present, whereas xylosylation alone did not cause IgE binding. CCD-specific IgG exhibited a dissociation constant of approximately 10(-8) mol/L. IgG(4) amounted to only 20% of the CCD-specific IgG (as well as total IgG). Conclusion: Low binding affinity of anti-CCD IgE cannot be the reason for the observed clinical insignificance of IgE against plant/insect glycan epitopes. Notably, the affinity of IgG to CCDs is higher than that to protein allergens, and it may therefore function as blocking antibody.