IgD receptors on murine T-helper cells bind to Fd and Fc regions of immunoglobulin D

Receptors for immunoglobulins on animal cells invariably show specificity for Fc regions of the protein and are hence called Fc receptors. The present study shows that immunoglobulin D receptors present an exception to this rule. Binding of IgD-coated erythrocytes to murine IgD-receptor-bearing T-helper cells is competitively inhibited by IgD, by its Fabδ fragments, and by deletion mutants of IgD lacking (i) the first constant domain of the δ heavy chain (KWD1), (ii) that region plus the δ heavy-chain-hinge region (KWD6), or (iii) the third constant domain of the δ heavy chain (Gen.24). KWD1, Gen.24, or KWD6 mutants bind to T-helper cells bearing receptors for IgD independently of each other. Furthermore, Gen.24 and KWD6 mutants also competitively inhibit binding of each other in cross-blocking experiments. These results show that the IgD receptor binds to the Fdδ and the Fcδ and cannot readily be explained by sequence homology between the two parts of the IgD molecule.