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VDJ recombination lets a finite genome produce more than 10⁶ distinct antibody specificities. It's one of biology's most elegant solutions to the combinatorial-diversity problem.
Antibody heavy chains are assembled from V (variable), D (diversity), and J (joining) gene segments — with imprecise junctions adding further diversity. Light chains use V and J only. Combinatorial assembly across multiple gene segments + somatic hypermutation in germinal centers + class switching between isotypes produces the breadth and specificity needed to cover any conceivable antigen.