The answer came as a surprise. A typical gene consists of a fixed segment of DNA, which directs the manufacture of a given protein molecule such as insulin. Antibody genes, in contrast, are assembled from bits and pieces of DNAscattered widely throughout the genetic materials. As the B cell matures, it rearranges or shuffles these gene components, picking and choosing among hundreds of DNA segments-some for each of the antibody's variable (V), diversity (D), joining (J), and constant (C) regions. Intervening segments of DNA are cut out; the selected pieces are spliced together.

The new gene-and the antibody it encodes-are virtually unique. When the B cell containing this uniquely rearranged set of gene segments proliferates, all its descendants will make this unique antibody. Then, as the cells continue to multiply, numerous mutants arise; these allow for the natural selection of antibodies that provide better and better "fits" for the target antigen. The result of this entire process is that a limited number of genetically distinct B cells can respond to a seemingly unlimited range of antigens.

A similar mechanism was found to control a comparable structure of the T cell, the T cell's antigen receptor. The variable regions of T cell antigen receptors, like those of antibodies, are encoded by V, D, and J segments originally far apart, but which are brought together and fused into a single gene. With numerous candidates for each segment, the number of possible combinations becomes astronomical. However, in contrast to antibody genes, T cell receptor genes do not mutate as the T cells proliferate. This ensures that the self-tolerance imposed in the thymus will not be overthrown by the inadvertent generation of mutant T cell receptors that are anti-self.

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