V(D)J recombination

V(D)J recombination is the unique mechanism of genetic recombination that occurs only in developing lymphocytes during the early stages of T and B cell maturation. It involves somatic recombination, and results in the highly diverse repertoire of antibodies/immunoglobulins (Igs) and T cell receptors (TCRs) found on B cells and T cells, respectively. The process is a defining feature of the adaptive immune system and its development was a key event in the evolution of jawed vertebrates.

V(D)J recombination occurs in the primary lymphoid organs (bone marrow for B cells and thymus for T cells) and in a nearly random fashion rearranges variable (V), joining (J), and in some cases, diversity (D) gene segments. The process ultimately results in novel amino acid sequences in the antigen-binding regions of Igs and TCRs that allow for the recognition of antigens from nearly all pathogens including bacteria, viruses, parasites, and worms as well as "altered self cells" as seen in cancer. The recognition can also be allergic in nature (e.g., to pollen or other allergens) or may be "autoreactive" and lead to autoimmunity.

In 1987, Susumu Tonegawa was awarded the Nobel Prize in Physiology or Medicine "for his discovery of the genetic principle for generation of antibody diversity".

Human antibody molecules (and B cell receptors) are composed of heavy and light chains (each of which contains both constant (C) and variable (V) regions), which are encoded by genes on three loci:

...
Wikipedia