THE MINOR IMMUNOGLOBULIN CLASSES: IgD AND IgE
IgD and IgE were the last immunoglobulins to be identified, due to their low concentrations in serum and low frequency of patients with multiple myeloma producing them. Both are monomeric immunoglobulins, similar to IgG, but their heavy chains are larger than chains. IgE has five domains in the heavy chain (one variable and four constant); IgD has four heavy-chain domains (as most other monomeric immunoglobulins).
IgD and IgM are the predominant immunoglobulin classes in the B-lymphocyte membrane, where they are the antigen-binding molecules in the antigen-receptor complex. Membrane IgD and IgM are monomeric. The heavy chains of membrane IgD and IgM (δ m, μm) differ from those of the secreted forms at their carboxyl termini, where the membrane forms have a hydrophobic transmembrane section and a short cytoplasmic tail, which are lacking in the secreted forms. In contrast, a hydrophilic section is found at the carboxyl ter-mini of heavy chains of secreted immunoglobulins. The membrane immunoglobulins form a membrane complex with several other membrane proteins, including Igα and Igβ , which have sequence motifs in their cytoplasmic portions that are required for signal transduction. No other biological role is known for IgD besides existing as a membrane immunoglobulin.
IgE has the unique property of binding to Fce receptors on the membranes of mast cells and basophils. The binding of IgE to those receptors has an extremely high affinity (7.7×109 I/M-1), about 100-fold greater than the affinity of IgG binding to monocyte re-ceptors. The high-affinity binding of IgE to basophil membrane receptors depends on the configuration of Ce3 and Ce4 domains and is the basis for the designation of IgE as ho-mocytotropic antibody. In allergic individuals, if those IgE molecules have a given anti-body specificity and react with the antigen while attached to the basophil or mast cell mem-branes, they will trigger the release of histamine and other substances, which cause the symptoms of allergic reactions .