Membrane IgD

In sharp contrast to the low levels of serum IgD, both in humans and in mice, IgD is expressed on the membrane of the majority of B lymphocytes. In mice, membrane IgD appears around the end of the first week postpartum, and the percentage of B cells with membrane IgD then rapidly increases, more or less in parallel with the maturation of the T cell system of the animal. However, 8 chains that are not expressed on the membrane are apparently synthesized quite early in developing B cells. A completely-rearranged 8 gene causes allelic exclusion in transgenic mice.

Membrane IgD and IgM molecules, with the same VH and with the same light chain, and therefore with the same antigen-binding site, are expressed together on the majority of B lymphocytes. Since p and 8 chains do not pair, there are no hybrid p/8 molecules. The ratio between the two isotypes may vary between high 8/low p and the reverse; it may be rather stable in some cells, like the Lyl-B cells, that are low 8 as a rule, or it may be variable even in single clones of B cells, like some human F.pstein-Barr virus (EBV)-transformed cells where it has been found to vary between the two extremes in the same clone. The ratio of IgM to IgD on a given B lymphocyte must have functional consequences that are not as yet totally clear; it is established, however, that membrane IgD is reduced in memory B cells and that it disappears with the process of maturation to plasma cells.

After cross-linking with anti-8 antibodies, mem brane IgD is internalized by the cells, much in the same way as the other membrane immunoglobulin isotypes; however, in some B cell lines, differences in the intracytoplasmic location of the IgD/anti-IgD complexes versus the IgM/anti-IgM complexes have been observed; this difference may have relevance to the role of the two isotypes in the processing of antigens by B lymphoid cells.

Membrane IgD molecules are associated with a heterodimer consisting of two proteins of 35 kDa (IgD-ct) and 39 kDa (IgD-|3); the presence of the IgD-ct chain is necessary for the membrane expression ot IgD. Membrane IgM is also associated with a heterodimer formed by the same IgD-[3 chain discussed above and by an IgM-specific IgM-ot chain ot 34 kDa. Thus the two membrane immunoglobulin isotypes are associated with companion molecules which are related but different; these biochemical observations may go a long way toward the understanding of the function of the two classes of anti gen receptors that coexist on the membrane of B lymphocytes.

So far, however, no gross differences have been detected between the biochemical events that take place in B cells after cross-linking IgM and those seen after cross-linking IgD; therefore, the immediate signals given to the B cell by the two classes of antigen receptors seem to be equivalent.

However, a difference in the kinetics of phos-

phorylation of the protein tyrosine kinases after cross-linking of IgM and IgD with the same antigen specificity has been detected by Kim and Reth. In IgM-expressing cells phosphorylation reaches a maximum in 1 min and decreases after 60 min, whereas in IgD cells the phosphorylation of the protein kinases reaches a maximum in 60 min and persists longer than 240 min after exposure to antigen. This may be relevant in conditions where the B cells are competing for antigen, as in the germinal centers.

How To Bolster Your Immune System

How To Bolster Your Immune System

All Natural Immune Boosters Proven To Fight Infection, Disease And More. Discover A Natural, Safe Effective Way To Boost Your Immune System Using Ingredients From Your Kitchen Cupboard. The only common sense, no holds barred guide to hit the market today no gimmicks, no pills, just old fashioned common sense remedies to cure colds, influenza, viral infections and more.

Get My Free Audio Book


Post a comment