IgD Receptors Research Articles

The Class of Surface Immunoglobulin on Virgin and Memory B Lymphocytes

The class of surface immunoglobulin receptors for antigen on B cell precursors of different classes of antibody-forming cells was determined by utilizing a technique of class-specific antigen suicide. Spleen cells are first treated with a class-specific antiserum under conditions that result in the stripping of that class from the cell surface. The cells are then permitted to bind a highly radioactive trinitrophenyl (TNP)-conjugated protein, which leads to lethal irradiation of all TNP-specific B cells except those whose TNP receptors had been removed by the class-specific stripping of surface immunoglobulin. In this way, the class of antibody-forming cells resulting from TNP stimulation of B cells with different classes of surface immunoglobulin can be examined. It was found that the virgin B cell precursors of IgM-producing cells are two types: cells bearing IgM receptors only and those bearing both IgM and IgD receptors. All virgin B cells that gave rise to IgG1 antibody-forming cells had both IgM and IgD on their surfaces, demonstrating that an antigen-dependent switch from IgM and IgD to IgG1 production is a common feature of B cell maturation. In contrast, memory B cell precursors of IgG1 antibody-forming cells had predominantly IgG1 as their surface antigen receptor. The implications of these findings on current models of B cell maturation are analyzed.

Structure and Biologic Functions of Human IgD

Abstract Chicken anti-human IgD antiserum (anti-δ) has demonstrated an antigenically cross-reactive homologue on rat lymphocytes. IgD and IgM are the only cell surface immunoglobulins detectable by the lactoperoxidase radiolabeling technique employed. The results indicate that, although rat surface IgD is antigenically distinct from rat IgM, the respective H chains co-electrophorese in 10% polyacrylamide-SDS gels. Rat δ-chain has an apparent m.w. of 73,000 daltons and exhibits a minor 65,000 dalton component which probably represents a partially degraded δ-chain. The ontogenic emergence of rat IgD occurs ∼3.5 weeks after birth whereas IgM, in contrast, is apparent by 6 days of age. Thus, as in the human, IgM develops before IgD. IgD receptors are undetectable in the thymus but are present in increasing levels in spleen, blood, lymph nodes, and Peyer's patches.

IgD receptors of lymphoid cells.

While present in normal human serum in very low amounts and undetectable in sera of non-human primates as well as of mice, IgD is found on the surface of the majority of B lymphocytes in all the above mentioned species. Lymphocytes which carry IgD on their membrane also have IgM. The two molecules are present in relative amounts that can be very different in different cells. Both IgM and IgD of a single cell are the actual product of the cell itself. They have the same light chain and, more importantly, the same combining site and idiotype. IgD/IgM bearing lymphocytes are the majority of all B lymphocytes in spleen, lymph nodes and Peyer's patches, whereas in bone marrow they account for half of the immunoglobulin positive cells. Although the percentage of double IgD/IgM cells is very similar in different tissues, the total amount of IgD, as well as the relative amounts of IgD and IgM as detected by biochemical methods varies. In fact, lymph nodes, and even more Peyer's patches are much richer than spleen in cells having levels of IgD higher than those of IgM; conversely, in the bone marrow, all the positive cells have very low levels of IgD. In ontogeny, as in evolution, IgD appears after IgM: in human foetuses IgD bearing cells are not detectable before 13 weeks of gestation, and in the mouse they appear only after birth. IgD receptors seem to disappear from B cells which undergo maturation to secretion, as indicated by the fact that only a proportion of IgM secreting plasma cells show membrane IgD. IgD is never found on the membrane of IgG-containing cells, and also lymphocytes bearing simultaneously IgD and IgG are very rare, and it might well be that for these cells, the double expression for short periods of time does not actually correspond to simultaneous synthesis.