Lymphocyte membrane, immunoglobulins

Loor, F., Fomi, L., Pemis, B. (1972). The dynamic state of the lymphocyte membrane. Factors affecting the distribution and turnover of surface immunoglobulins. Eur. J. Immunol. 2,203-212. 

A B-lymphocyte can simultaneously express membrane immunoglobulin as IgM (monomeric) or IgD with the same variable region (i.e., antigen-binding site). The B-lymphocyte then can secrete different isotypes [e.g., IgM... 

Corticosteroids stabilize lymphocytic membranes, reducing the production of immunoglobulins and their compliment. Lymphocyte production is also inhibited, producing lymphocytopenia. 

In contrast to human membrane-derived immunoglobulins M and D, those from murine lymphocyte membrane display different affinities for lentil lectin. 

Affinity chromatography of surface glycoproteins and immunoglobulin E receptors from rat basophilic leukaemia cells Affinity chromatography of membrane immunoglobulin M and murine lymphocyte immunoglobulin study of their affinities for the lectin... 

Whatever the mode of attachment of SIg to cell membranes, the major portion of the Fc part of the molecule is not embedded in the phospholipid bilayer. This is also supported by the fact that CD spectra of all membrane proteins (not lymphocytes specifically) show little conformation, which, as previously mentioned, is the conformation of immunoglobulin domains. Experiments with reconstituted lymphocyte plasma membranes (Chavin and Holliman, 1975) and model membrane systems (Weissman et al., 1974) have not, in general, been very revealing. With reconstituted membranes, very little of the original membrane immunoglobulin becomes reincorporated. Liposome models were tested by... 

When B lymphocytes are stimulated by antigen they develop into plasma cells. The receptor of plasma cells can be secreted from the cell because some of these molecules lack the hydrophobic domain that normally anchors the receptor to the plasma membrane. These secreted molecules are immunoglobulins or antibodies and have a common structure (Fig. 1.8). They are found in a variety of bodily fluids and in many tissues. 

Figure 17.44 Some factors that are required for completion of the cell cycle in lymphocytes. The cell cycle is described in detail in chapter 20. The initial signal is binding of the antigen to the receptor immunoglobulin in the membrane of the B cell.

Immunoglobulin M (IgM), a membrane protein on the surface of B lymphocytes, serves to bind free antigens to the B cells. By contrast, T cell receptors only bind antigens when they are presented by another cell as a complex with an MHC protein (see below). Interaction between MHC-bound antigens and T cell receptors is supported by co-recep-tors. This group includes CDS, a membrane protein that is typical in cytotoxic T cells. T helper cells use CD4 as a co-receptor instead (not shown). The abbreviation CD stands for cluster of differentiation. It is the term for a large group of proteins that are all located on the cell surface and can therefore be identified by antibodies. In addition to CD4 and CDS, there are many other co-receptors on immune cells (not shown). 

Examples of antibodies Many Abs to toxins, bacteria, viruses especially late in Ab response. Many Abs to infectious agents, especially early in Ab response antipolysaccharide Ab cold agglutinins. Important as secretory antibody on mucous membranes. No proved Ab activity main immunoglobulin on surface of (8 lymphocytes in newborn. Binds to mast and basophil cells raised in allergic and parasitic infections. 

T-cell activation involves interactions of membrane-bound immunoglobulin-like receptors with membranous antigen-MHC complexes. The antigen receptors of B and T lymphocytes are complex membrane-bound multisubunit proteins. Antigen receptors are immunoglobulin (Ig)-like antibody molecules. B cells do not secrete the first antibodies that they make, instead, they insert them into the plasma membrane, where they now serve as receptors for antigen. Each B cell has approximately 10 such molecules in the plasma membrane. 

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