Human IgD and IgA1 compete for D-galactose-related binding sites on the lectin jacalin.

Lectin selectivity for human Ig classes is based on carbohydrate differences. Earlier reports that the lectin jacalin precipitated human IgA were confirmed and supplemented by the current study, which demonstrates that jacalin also binds human IgD as evaluated by micro-ELISA and SDS-PAGE. Experimental findings indicated that: (i) Monoclonal and polyclonal (sera) IgD, IgA1, but not IgA2, IgM, or IgG1-4 reacted with jacalin. (ii) Six tested monoclonal IgD proteins each bound approximately equally to jacalin when antigenicity rather than protein concentration was measured: the results weigh against the presence of jacalin-detectable IgD subclasses or genetic variants. (iii) IgD and IgA1 both associated maximally in 4-8 h at 4 degrees C. There was no dissociation at 4 degrees C but limited dissociation occurred at 37 degrees C after 24 h. (iv) Both IgD and IgA1 were eluted from jacalin by galactose-related sugars. (v) IgD and IgA1 bind competitively to jacalin. The results suggested that jacalin reacts with O-linked oligosaccharide N-acetyl-galactosamine (GalN) residues found on the hinge region of both IgD and IgA1. Jacalin also interacted with one major and several minor unidentified sera proteins. The findings offer an approach to the isolation of serum polyclonal IgD and to the characterization of the unusual carbohydrates of the human delta heavy chain with respect to their function.