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Immunology · Adaptive immunity

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

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

  1. A developing lymphocyte opens the chromatin of one receptor locus (in B cells the immunoglobulin heavy chain goes first).
  2. RAG1/RAG2 binds recombination signal sequences (RSS) flanking two segments, pairing one with a 12-bp spacer and one with a 23-bp spacer.
  3. RAG cuts the DNA at the segment borders; the coding ends are sealed as hairpins and the DNA between them is removed.
  4. Artemis opens the hairpins and TdT adds random N nucleotides, so each junction is different (junctional diversity).
  5. Non-homologous end joining enzymes (DNA ligase IV and partners) ligate the ends: D joins J, then V joins DJ.
  6. 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).
  7. The light chain then rearranges (kappa first, then lambda); a complete IgM is the receptor of the immature B cell.
  8. 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.

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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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 (U.S. National Library of Medicine (MedlinePlus), 2026); 18.4 B Lymphocytes and Humoral Immunity (OpenStax (Microbiology), 2016); 18.3 T Lymphocytes and Cellular Immunity (OpenStax (Microbiology), 2016); 19.4 Immunodeficiency (OpenStax (Microbiology), 2016).

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Representative model, for educational purposes only.