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 .
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