Roughly forty variable, twenty-three diversity, and six joining gene segments, cut and rejoined by a two-protein enzyme complex, generate an antibody repertoire theoretically capable of exceeding 10¹³ distinct specificities. This chapter states the real, IMGT-sourced segment counts (correcting an earlier placeholder figure in this book's own index), the real diversity estimate, and the mechanism — RAG1/RAG2-mediated site-specific recombination — without needing any embellishment to be remarkable.
B cells and T cells assemble their antigen receptors from separate variable (V), diversity (D), and joining (J) gene segments, which sit apart from each other in the genome and are cut and spliced together during lymphocyte development. The enzyme complex RAG1–RAG2 initiates this by cutting DNA at recombination signal sequences (RSSs) flanking each segment; the cut ends are then rejoined by the cell's non-homologous end-joining machinery, and imprecision at the joins (P-nucleotide and N-nucleotide addition, catalyzed in part by terminal deoxynucleotidyl transferase, TdT) adds further variability beyond the segment choice itself.
The correction matters because "10⁷" undersells the real number by four to six orders of magnitude — the genuinely striking fact here is not a modest seven-digit repertoire, it is a combinatorial and junctional process that can in principle exceed the number of B cells a person will ever have.
VDJ recombination is best read through this corpus's C (Contact) operator, generalized from a single binding event to a combinatorial selection among many possible segment choices: RAG1/RAG2 establishes contact with RSS sites across the V, D, and J segment pools, and which specific segments get joined is the combinatorial "contact" event that seeds everything downstream (Ch 13's thymic selection, Ch 14's germinal-center affinity maturation). This chapter does not extend the mapping further than that single, honest correspondence — the recombination event is a contact/selection step, not a threshold, fold, or unfold event in this corpus's sense, and forcing it into all four operator slots would not add anything true.
Schatz, D.G., Swanson, P.C. (2011). V(D)J recombination: mechanisms of initiation. Annu. Rev. Genet. 45, 167–202.
IMGT®, the international ImMunoGeneTics information system: imgt.org — source for the functional segment counts in §1.
Glanville, J. et al. (2009). Precise determination of the diversity of a combinatorial antibody library gives insight into the human immunoglobulin repertoire. PNAS 106, 20216–20221. — Source for the realized/theoretical diversity estimates in §1.
See also: Ch 13 — Thymic Selection · Ch 14 — Germinal Centres · Book VI Index