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# V(D)J recombination: how a few genes make billions of receptors

- Process

Each B and T cell cuts and rejoins pieces of its own DNA to build a one-of-a-kind receptor. Without the cutting enzymes (RAG) there are no B or T cells.

Updated 2026-11-01

## Key points

- RAG1/RAG2 cut DNA at RSS sequences; only developing lymphocytes express them.
- The 12/23 rule makes segments join in the right order (V to D to J).
- TdT adds N nucleotides at junctions; it is also a marker of lymphoblastic leukaemia (immature lymphocytes).
- RAG loss gives T-B-NK+ SCID; hypomorphic RAG gives Omenn syndrome.
- Allelic exclusion: each B cell expresses a receptor of a single specificity.
- TRECs on the newborn blood spot screen for SCID.

## What it is

V(D)J recombination is the DNA cut-and-join process that assembles the variable part of antibodies (B-cell receptors) and T-cell receptors from separate V, D and J gene segments. It happens only in developing lymphocytes, in the bone marrow (B cells) and thymus (T cells), and it is driven by the RAG1/RAG2 enzyme complex. Random segment choice plus the imprecise joining of the ends lets a small number of genes produce a potential repertoire on the order of 10^11 different receptors. Defects in the machinery cause severe combined immunodeficiency (SCID).

## Key facts

- Heavy chains and TCR beta chains use V, D and J segments; light chains and TCR alpha chains use only V and J.
- Combinatorial diversity (which segments) plus junctional diversity (nucleotides added or lost at the joins) plus chain pairing create the repertoire; junctional diversity contributes the most.
- The junctions encode CDR3, the most variable loop and the main contact with antigen.
- T cells follow the same logic in the thymus: TCR beta rearranges first and pairs with pre-T alpha; TCR alpha rearranges later.
- Somatic hypermutation and class switching happen later, in germinal centres, and use AID, not RAG.

## Step by step

- A developing lymphocyte opens the chromatin of one receptor locus (in B cells the immunoglobulin heavy chain goes first).
- RAG1/RAG2 binds recombination signal sequences (RSS) flanking two segments, pairing one with a 12-bp spacer and one with a 23-bp spacer.
- RAG cuts the DNA at the segment borders; the coding ends are sealed as hairpins and the DNA between them is removed.
- Artemis opens the hairpins and TdT adds random N nucleotides, so each junction is different (junctional diversity).
- Non-homologous end joining enzymes (DNA ligase IV and partners) ligate the ends: D joins J, then V joins DJ.
- A productive heavy chain pairs with a surrogate light chain as the pre-B receptor, which signals survival and stops the other allele (allelic exclusion).
- The light chain then rearranges (kappa first, then lambda); a complete IgM is the receptor of the immature B cell.
- Self-reactive immature B cells get a second chance by receptor editing; the rest die. Mature naive B cells leave expressing IgM and IgD.

## Markers

- RAG1
- RAG2
- TdT
- Artemis
- DNA ligase IV
- RSS 12/23
- CD19
- CD3

## Clinical notes

- Complete RAG1 or RAG2 loss: SCID with no T and no B cells but normal NK cells (T-B-NK+), infections from the first months of life.
- Partial (hypomorphic) RAG function: Omenn syndrome, with erythroderma, eosinophilia, high IgE, enlarged nodes and liver, and a few oligoclonal autoreactive T cells.
- Artemis or DNA ligase IV defects also cause T-B- SCID, with added sensitivity to ionising radiation.
- Newborn screening by TREC (T-cell receptor excision circles, the DNA circles cut out during TCR recombination) detects SCID before infections start.
- Clonality testing of rearranged receptor genes helps distinguish lymphomas and leukaemias (one clone) from reactive proliferations (many clones).

## In the lab

- TREC count on the newborn blood spot (low or absent in SCID).
- Flow cytometry for T (CD3), B (CD19) and NK (CD16/CD56) cells defines the SCID phenotype.
- PCR of IGH or TCR gene rearrangements for clonality in lymphoid neoplasms.

## Memory aid

Think of a jukebox with three record racks: RAG picks one V, one D and one J, and TdT scratches the record a little at each join so no two songs sound the same.

## Test yourself

Think of the answer, then tap the question to see it.

## Which enzyme complex starts V(D)J recombination by cutting DNA at the recombination signal sequences?

RAG1/RAG2, expressed only in developing B and T cells.

## A baby has no T cells and no B cells but normal NK cells. Which genes are typical culprits?

RAG1 or RAG2 (also Artemis or DNA ligase IV): defects of V(D)J recombination give T-B-NK+ SCID.

## Which enzyme adds random nucleotides at the joins and creates junctional diversity?

Terminal deoxynucleotidyl transferase (TdT).

## What does the 12/23 rule ensure?

That a segment flanked by a 12-bp spacer RSS joins only one flanked by a 23-bp spacer, so V, D and J join in the right order.

## Erythroderma, eosinophilia, high IgE and lymphadenopathy in an infant with partial RAG function suggest which syndrome?

Omenn syndrome, a leaky form of SCID with a few oligoclonal autoreactive T cells.

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## Related

- [B cell and plasma cell](https://durgical.com/en/immunology/b-cell-and-plasma-cell/)
- [T-cell development: positive and negative selection](https://durgical.com/en/immunology/t-cell-development-positive-and-negative-selection/)
- [Antibody structure and isotypes (IgG, IgA, IgM, IgE, IgD)](https://durgical.com/en/immunology/antibody-structure-and-isotypes/)
- [Fc receptors, BCR and TCR](https://durgical.com/en/immunology/fc-receptors-bcr-and-tcr/)
- [T-cell and combined deficiencies: DiGeorge, SCID, Wiskott–Aldrich, ataxia-telangiectasia](https://durgical.com/en/immunology/t-cell-and-combined-deficiencies-digeorge-scid-wiskott-aldrich-ataxia-telangiect/)
- [Immune tolerance and how autoimmunity begins](https://durgical.com/en/immunology/immune-tolerance-and-how-autoimmunity-begins/)

## Related conditions

- [Adenosine deaminase deficiency (ADA-SCID)](https://durgical.com/en/conditions/adenosine-deaminase-deficiency/)

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Educational content on the immune system; not a treatment or individual vaccination guide and no substitute for a health professional's advice. No doses are given.

Sources: [RAG1 gene: MedlinePlus Genetics](https://medlineplus.gov/genetics/gene/rag1/) (U.S. National Library of Medicine (MedlinePlus), 2026); [18.4 B Lymphocytes and Humoral Immunity](https://openstax.org/books/microbiology/pages/18-4-b-lymphocytes-and-humoral-immunity) (OpenStax (Microbiology), 2016); [18.3 T Lymphocytes and Cellular Immunity](https://openstax.org/books/microbiology/pages/18-3-t-lymphocytes-and-cellular-immunity) (OpenStax (Microbiology), 2016); [19.4 Immunodeficiency](https://openstax.org/books/microbiology/pages/19-4-immunodeficiency) (OpenStax (Microbiology), 2016).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/immunology/v-j-recombination-how-a-few-genes-make-billions-of-receptors/