Antibody diversity is generated by the following mechanisms.
1) The joining of various V, D and J genes is entirely random that results in ~ 50,000 different possible combinations for VDJ(H) and ~ 1,000 for VJ(L). Subsequent random pairing of H and L chains brings the total number of antibody specificities to ~107 possibilities. 2) Diversity is further increased by the imprecise joining of different genetic segments. 3) Rearrangements occur on both DNA strands, but only one strand is transcribed (allelic exclusion). 4) Only one rearrangement occurs in the life of a B cell because of irreversible deletions in DNA. Consequently, each mature B cell maintains one immunologic specificity and is maintained in the progeny or clone. This constitutes the molecular basis of the clonal selection; i.e., each antigenic determinant triggers the response of the pre-existing clone of B lymphocytes bearing the specific receptor molecule. It also follows that deletion of the B cell clone results in immunologic unresponsiveness to the antigen.
This mechanism leads to a fine-tuning of the antibody specificity after immunization. Rearranged VDJH, and VJL genes in the B cells are uniquely susceptible to point mutagenesis by enzymes that become activated following stimulation of the cell by antigen. The clonal progeny of an antigen-driven B cell thus produce antibodies that may differ in one or more amino acid positions in the regions of the protein that are responsible for antigen binding. Cells producing the mutant antibody with highest affinity for the antigen are preferentially stimulated and thus eventually dominate the response. Therefore, antibodies produced after repeated immunization commonly display numerous point mutations (derived by somatic mutations in B cells found in peripheral lymphoid organs) and have higher affinities for antigens (affinity maturation), as compared to antibodies produced in the primary immune response.
Antibody molecules perform a number of important functions that are necessary for mounting an effective immune response against microbial pathogens. CH region genes encode the biological functions of immunoglobulins (Table 1-3). For example, IgM and IgG bind to the C1q subunit of C1, IgG crosses the placental barrier to the fetal circulation, and polymeric immunoglobulins, particularly dimeric IgA, are transported across epithelial cells into mucosal secretions.
To accomplish these functions, B cells switch their immunoglobulin isotype. The VDJ genes which are associated with Cμ or Cδ, which are the original constant genes expressed in mature B cells, become associated with another C gene (). This has been termed the isotype switch, because the C gene determines the antibody isotype. The switch is accomplished by genetic recombination, whereby the VDJ gene segment is transferred from the Cμ/Cδ junction onto another C region gene downstream (Fig. 1-11). Because the Cμ/Cδ and other interposed genes are deleted, the switch is irreversible. The new antibody maintains the same L chain and the same VH region (encoded by VDJ), but has new properties determined by the acquired C gene. The isotype switch mechanism is promoted by physical interactions between T and B cells (for example, the binding of CD40 on B cells to its ligand on T cells) and by specific cytokines from T cells (for example, IL-5 and IL-10 promote IgA production; IL-4 promotes IgE production).
Furthermore, each antibody molecule may exist in either a membrane-bound or secreted form. Every C gene contains a 3′ sequence encoding the hydrophobic cytoplasmic tail of the H chain, so that the immunoglobulin molecule produced by the B cell is inserted in the surface membrane to function as the receptor for antigen. When the B cell differentiates into a plasma cell, an enzyme is activated that modifies the RNA transcript. Consequently, the translated protein ends with a hydrophilic peptide and is secreted from the cell.
To accomplish these functions, B cells switch their immunoglobulin isotype. The VDJ genes which are associated with Cμ or Cδ, which are the original constant genes expressed in mature B cells, become associated with another C gene (). This has been termed the isotype switch, because the C gene determines the antibody isotype. The switch is accomplished by genetic recombination, whereby the VDJ gene segment is transferred from the Cμ/Cδ junction onto another C region gene downstream (Fig. 1-11). Because the Cμ/Cδ and other interposed genes are deleted, the switch is irreversible. The new antibody maintains the same L chain and the same VH region (encoded by VDJ), but has new properties determined by the acquired C gene. The isotype switch mechanism is promoted by physical interactions between T and B cells (for example, the binding of CD40 on B cells to its ligand on T cells) and by specific cytokines from T cells (for example, IL-5 and IL-10 promote IgA production; IL-4 promotes IgE production).
Furthermore, each antibody molecule may exist in either a membrane-bound or secreted form. Every C gene contains a 3′ sequence encoding the hydrophobic cytoplasmic tail of the H chain, so that the immunoglobulin molecule produced by the B cell is inserted in the surface membrane to function as the receptor for antigen. When the B cell differentiates into a plasma cell, an enzyme is activated that modifies the RNA transcript. Consequently, the translated protein ends with a hydrophilic peptide and is secreted from the cell.