one variable (V), one diversity (D), and one joining (J) region for genetic recombination and discards the rest, which produces a highly unique combination of antigen-receptor gene segments in each lymphocyte. This mechanism allows a small number of genetic segments to generate a vast number of different antigen receptors, which are then uniquely expressed on each individual lymphocyte. Since the gene rearrangement leads to an irreversible change in the DNA of each cell, all progeny (offspring) of that cell inherit genes that encode the same receptor specificity, including the memory B cells and memory T cells that are the keys to long-lived specific immunity. Naming The term "adaptive" was first used by Robert Good in reference to antibody responses in frogs as a synonym for "acquired immune response" in 1964. Good acknowledged he used the terms as synonyms but explained only that he preferred to use the term "adaptive". He might have been thinking of the then not implausible theory of antibody formation in which antibodies were plastic and could adapt themselves to the molecular shape of antigens, and/or to the concept of "adaptive enzymes" as described by Monod in bacteria, that is, enzymes whose expression could be induced by their substrates. The phrase was used almost exclusively by Good and his students and a few other immunologists working with marginal organisms until the 1990s when it became widely used in tandem with the term "innate immunity" which became a popular subject after the discovery of the Toll receptor system in Drosophila, a previously marginal organism for the study of immunology. The term "adaptive" as used in immunology is problematic as acquired immune responses can be both adaptive and maladaptive in the physiological sense. Indeed, both acquired and innate immune responses can be both adaptive and maladaptive in the evolutionary sense. Most textbooks today, following the early use by Janeway, use "adaptive" almost exclusively and noting in glossaries that the term is synonymous with "acquired". The classic sense of "acquired immunity" came to mean, since Tonegawa's discovery, "antigen-specific immunity mediated by somatic gene rearrangements that create clone-defining antigen receptors". In the last decade, the term "adaptive" has been increasingly applied to another class of immune response not so-far associated with somatic gene rearrangements. These include expansion of natural killer (NK) cells with so-far unexplained specificity for antigens, expansion of NK cells expressing germ-line encoded receptors, and activation of other innate