Junctional diversity

Junctional diversity is a mechanism in the immune system that contributes to the generation of a vast repertoire of antibodies and T-cell receptors. This process occurs during the recombination of gene segments in the development of B cells and T cells.

Mechanism

Junctional diversity arises during the process of V(D)J recombination, which is the rearrangement of variable (V), diversity (D), and joining (J) gene segments in the immunoglobulin and T-cell receptor genes. This recombination process is mediated by the RAG1 and RAG2 proteins, which introduce double-strand breaks at specific recombination signal sequences flanking the V, D, and J segments.

During the joining of these segments, additional diversity is introduced at the junctions through several mechanisms:

• **N-nucleotide addition**: The enzyme terminal deoxynucleotidyl transferase (TdT) adds random nucleotides to the ends of the gene segments.
• **P-nucleotide addition**: Palindromic sequences are created by the asymmetric cleavage of hairpin loops formed at the coding ends.
• **Exonuclease trimming**: Nucleotides may be removed from the ends of the gene segments before they are joined.

These processes result in the addition or deletion of nucleotides at the junctions, creating unique sequences that contribute to the diversity of the antigen-binding sites of antibodies and T-cell receptors.

Importance

Junctional diversity is crucial for the adaptive immune system's ability to recognize a vast array of antigens. The unique sequences generated at the junctions of V, D, and J segments allow for the production of a wide variety of antibodies and T-cell receptors, each with a different specificity. This diversity enables the immune system to respond to a multitude of pathogens and provides the basis for immunological memory.

Related Concepts

• Somatic hypermutation
• Class switch recombination
• Antigen presentation
• Major histocompatibility complex
• Clonal selection theory

See Also

• Adaptive immune system
• B cell receptor
• T cell receptor
• Immunoglobulin gene