Epitope analysis of human immunoglobulin D with a mouse monoclonal anti-Fab delta chain antibody.

Background: The clinical significance of IgD measurements in serum is limited as the frequency of abnormal concentrations is rare.
Methods: We prepared a mouse monoclonal antibody and a rabbit polyclonal antibody against Fab delta and Fc delta chain and compared epitope recognition by the monoclonal antibody against Fab delta (anti-Fab delta mono) with that by other antibodies.
Results: Anti-Fab delta mono specifically reacted with purified IgD and Fab delta of myelomatous origin, but not with other isotypes and light chains. In 19 of 22 myeloma sera, the monoclonal antibody recognized intact IgD and/or its fragments when analyzed by immunoblotting. Of these there were only four cases in which possible Fab delta fragments were identified. The other three sera showed no reactivity with the antibody and the IgD value was low on a chemiluminescence enzyme immunoassay.
Conclusions: These findings indicate the presence of at least two immunochemically different IgD molecules in the sera. No positive reaction with any synthetic peptide was observed for the antibody on delta-chain ranging from N-terminus of JH, C delta 1 to a hinge region. We suggest that the epitope recognized by the antibody is related to the variable region or a conformational structure on the Fd delta region of IgD.