Genetic Basis of Antibody Structure
Every individual is capable of producing an enormous diversity of immunoglobulin (Ig) molecules, each acting as a B-cell receptor that recognizes a specific epitope. This structural diversity is created through unique genetic mechanisms that assemble the variable regions of immunoglobulin chains.
Organization of Ig Genes
The variable region of the heavy chain is encoded by three separate gene segments:
- VH (Variable)
- DH (Diversity)
- JH (Joining)
A separate CH gene segment encodes the constant region of the heavy chain.
The variable region of the light chain is encoded by:
- VL (Variable)
- JL (Joining)
Light chains also have their own constant region gene segments (Cκ or Cλ). Every nucleated cell contains multiple germline V, D and J segments for both heavy and light chain formation.
Gene Rearrangement
During B-cell differentiation:
- Heavy chain DNA is rearranged so that one VH segment joins one DH segment and one JH segment, forming a VDJ unit.
- This VDJ unit is placed next to the constant region genes Cμ and Cδ.
- Light chain genes undergo a similar process, joining one VL segment to one JL segment.
- In κ-expressing cells, VκJκ is placed next to Cκ.
- In λ-expressing cells, VλJλ is placed next to Cλ.
Once rearrangement occurs, the antigenic specificity of that B cell becomes permanently fixed.
Primary RNA transcripts are produced from the rearranged DNA, unnecessary RNA is removed by splicing, and the final mRNA encodes light and heavy chains of IgM and IgD.
Class Switching
After antigen exposure, a B cell can undergo class switching.
The VDJ unit originally linked to Cμ and Cδ can recombine with other heavy-chain constant region genes such as Cγ, Cα, or Cε. This allows the B cell to produce IgG, IgA, or IgE, all with the same antigen specificity but different effector functions.
Mechanisms Generating Antibody Diversity
Antibody specificity diversity is generated through multiple mechanisms:
- Multiple inherited V-region genes for both heavy and light chains
- Combinatorial rearrangement of V, D, and J segments
- Random pairing of heavy and light chains
- Junctional and insertional diversity during joining of gene segments
- Somatic hypermutation after antigen exposure, allowing selection for mutations that increase affinity
Together, these mechanisms allow a relatively small number of genes to produce an enormous range of antibodies with different specificities.