The Fragmentation Principle and Solving the Genetic Problem
For a long time, immunology faced a coding dilemma: the number of inherited structural (germline) genes is negligible compared to the incredible diversity of existing antibodies. The solution turned out to be the fragmentation principle.
In DNA, immunoglobulin genes are not presented as continuous stretches. They are scattered across the chromosome as individual fragments (segments). Each such gene encodes only a specific segment of the antibody molecule. To produce a functional, working gene, the cell must assemble these fragments together.
This assembly is called recombination (or rearrangement). It occurs during the maturation of the B lymphocyte while it prepares to become a plasma cell. Thanks to enzymes called recombinases, which cleave and ligate DNA strands, the segments combine in a completely random manner. As a result, each individual B cell forms its own unique gene sequence.
Sources of Antibody Diversity
The historical discovery of the molecular mechanisms of DNA rearrangement belongs to Susumu Tonegawa. According to his concept, the massive variability of immunoglobulins is ensured by three main factors:
- Combinatorial diversity (DNA segment recombination) — baseline shuffling of available gene fragments.
- Junctional inaccuracy — during segment joining, "extra" nucleotides may be randomly added at their junctions, altering the final protein structure.
- Somatic hypermutations — point mutations occurring within V genes (variable regions).
Organization and Assembly of Heavy (H) Chains
The genetic material for heavy chains is located on chromosome 14. In an immature cell, this locus is divided into four key regions:
- V (variable) — about 50 segments responsible for variability.
- D (diversity) — approximately 30 segments providing additional diversity.
- J (joining) — 6 joining segments.
- C (constant) — the constant region, represented by 9 genes that determine the class of the future antibody (e.g., the $\mu$ gene encodes IgM, and $\gamma1$ encodes IgG1).
The H-chain rearrangement process occurs in stages. First, D-J recombination takes place: a D gene joins a J gene, and the intervening DNA is deleted. Next, a V gene (along with a signaling L segment) attaches to this DJ complex. The formed sequence is transcribed into mRNA, which also includes the constant gene (initially $C\mu$).
After translation, a protein with a leader sequence is formed. When enzymes cleave this signaling L peptide, the formation of the final heavy $\mu$ chain is complete. Notably, this entire process involves active splicing (removal of non-coding introns) and occurs strictly before contact with a real antigen.
Specifics of Light Chains
The genetics of light chains differ in both chromosomal localization and locus structure. There are two types of light chains:
- $\kappa$ (kappa) chains: The genes reside on chromosome 2. They contain about 40 functional V genes and 5 J genes (which encode additional amino acids). During assembly, a V gene relocates adjacent to a J gene, followed by co-transcription with the constant region gene ($C\kappa$).
- $\lambda$ (lambda) chains: Located on chromosome 22. Their primary architectural feature is the presence of multiple constant (C) region genes, where J sequences are not arranged as a single common block, but rather directly preceding each individual C gene.
Immunoglobulin Class Switching
Mature B lymphocytes initially produce IgM class antibodies. However, during an immune response, the cell often needs to switch the class of the synthesized antibody (e.g., to IgG or IgA) while preserving its antigen specificity. This phenomenon is called isotype switching (class switch recombination).
The mechanism relies on the already rearranged VDJ complex (responsible for recognition) joining a different heavy chain C gene located further downstream on the DNA strand.
A key role is played by S regions (switch regions) located upstream of each C gene (except $C\delta$). These regions possess a high degree of homology, allowing them to readily recombine with one another. During isotype switching (e.g., from IgM to IgG1), the DNA segment between the VDJ complex and the new target C gene (including the old $C\mu$, $C\delta$, and other genes) is permanently excised and deleted from the cell's genome.