Total serum IgD is increased in atopic subjects

We have developed a sandwich‐type ELISA system for measuring total IgD levels in the serum of atopics and non‐atopic controls. In this ELISA system, affinity purified goat anti‐human IgD was used for capture. Results were superior to those obtained with monoclonal anti‐human IgD antibody. No cross‐reactivity could be demonstrated to IgG. IgM, IgA or IgE. The assay showed minimal non‐specific binding even with initial serum dilutions of 1:2. The results obtained were reproducible among replicates (Mean CV ± SEM = 0.03 ± 0.002; n= 251), between dilutions (CV = 0.08 ± 0.006; n= 108), and between assays (CV = 0.05 ± 0.12; n= 5). We used routine radioimmunoassay for measuring total serum IgE. Using these assays total serum IgD and IgE levels were measured in 75 atopic patients and 33 normal subjects. None of the atopics had recent immunotherapy. As expected, the geometric mean serum IgE in atopics (373 ng/ml) was significantly higher than that in normal subjects (49 ng/ml) (P < 0.01). However, geometric mean serum IgD was also significantly higher in atopics (20.3 μg/ml) than that in normal subjects (8.4 μg/ml) (P < 0.02). In both atopic and normal groups, mean serum IgD level did not differ significantly on the bases of age, sex or asthmatic status. Furthermore, total serum IgD was not significantly correlated with total serum IgE (r = 0.14; P= 0.14; n= 108), indicating that immunoregulatory control of the basal levels of the two isotypes is not linked. In a subset of more thoroughly studied atopic patients, total serum IgD but not serum IgE was correlated with D₂O‐induced basophil histamine release (r = 0.34: P < 0.05; n= 37). There was no correlationship of serum IgD or serum IgE to methacholine inhalation challenge, to spontaneous basophil histamine release or that induced by anti‐IgE, calcium icnophore, or a platelet‐derived histamine releasing factor (PD‐HRF). However, among poor responders to PD‐HRF, serum IgD was also correlated with PD‐HRF induced histamine release (r = 0.59; P < 0.025; n= 21). Our preliminary results indicate that the role of IgD in atopic disorders deserves further investigation.