GENERATION OF ANTIBODY DIVERSITY

The immune system has the capacity to recognize and respond to about 10⁷ different antigens. This extreme diversity can be generated in at least three possible ways:

1. Multiple genes in the germ line DNA.
2. Variable recombination during the differentiation of germ line cells into B-cells.
3. Mutation during the differentiation of germ line cells into B-cells.

It is known that all three of these possibilities take place to produce antibody diversity. The following figures illustrate these possibilities:

1. The figure shows the genetic makeup of a germ line cell and a mature B-cell at the loci controlling heavy chain production. Germ line DNA has many (up to 200) different variable (V) region genes, in addition to 12 diversity (D) region genes and four joining (J) region genes. During differentiation of this cell into the B-cell, rearrangement of the DNA occurs. This rearrangement aligns one of the many V genes with one of the D genes and one of the J genes, producing a functional VDJ recombinant gene. Since any of the genes may recombine with any others, this rearrangement has the potential to generate 200 x 12 x 4 = 9600 different possible combinations. The same type of event occurs in the genes encoding the immmunoglobulin light chains where about 200 different V regions may recombine with about 5 different J regions giving rise to 200 x 5 = 1000 possible light chains. Since in any particular B-cell, any light chain combination can occur along with any heavy chain combination, the total possible immunoglobulin combinations approaches 10⁷ (9600 x 1000).

   

2. A second way that diversity can result is through a process of variable or "inaccurate" recombination. The figure illustrates three possible recombination events between the variable (V) and joining (J) regions of an immunoglobulin light chain. In the first event, a proline-tryptophan dipeptide sequence is produced in the resulting protein. However, in the second and third events, differential recombination places proline-arginine or proline-proline sequences into the resulting immunoglobulin. These types of events may also occur between the V and D regions and the D and J regions of the heavy chain DNA sequence.

   

3. A third way that diversity can result is through a process of mutation. This process simply involves changes in DNA sequence that occur during differentiation of the B-cell. The figure illustrates how an A:T to G:C transition mutation could change a serine residue into a glycine residue in the resulting immunoglobulin. This process may, in part, explain the diversity observed in hypervariable (CDR) regions.

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IMMUNOGLOBULIN PRODUCTION

The production of immunoglobulins by B-cells or plasma cells occurs in different stages. During differentiation of the B-cells from precursor stem cells, rearrangement, recombination and mutation of the immunoglobulin V, D, and J regions occurs to produce functional VJ (light chain) and VDJ (heavy chain) genes. At this point, the antigen specificity of the mature B-cell has been determined. Each cell can make only one heavy chain and one light chain, although the isotype of the heavy chain may change. Initially, a mature B-cell will produce primarily IgD (and some membrane IgM) that will migrate to the cell surface to act as the antigen receptor. Upon stimulation by antigen, the B-cell will differentiate into a plasma cell expressing large amounts of secreted IgM. Some cells will undergo a "class switch" during which a rearrangement of the DNA will occur, placing the VDJ gene next to the genes encoding the IgG, IgE or IgA constant regions. Upon secondary induction (i.e. the secondary response), these B-cells will differentiate into plasma cells expressing the new isotype. Most commonly, this results in a switch from IgM (primary response) to IgG (secondary response). The factors that lead to production of IgE or IgA instead of IgG are not well understood.