The evolution of adaptive immunity.
The concept of adaptive immunity suggests de novo generation in each individual of extremely large repertoires of diversified receptors and selective expansion of receptors that match the antigen/pathogen. Accordingly, adaptive immune system is also called "anticipatory". It allows each individual to have a unique repertoire of immune receptors corresponding to its life history. The memory of an antigen gets encoded in the clonal composition of the organism's immune cells instead of being encoded in the genome. Consequently, the immune response to repeated encounter with the same antigen becomes stronger, a phenomenon called immunological memory. Elements of adaptive immunity are found at all taxonomical levels, whereas in vertebrates, adaptive mechanisms have become the cornerstone of the immune system. In jaw vertebrates, adaptive immune receptors of T and B lymphoid cells belong to immunoglobulin superfamily and are created by rearrangement of gene segments. In jawless vertebrates lamprey and hagfish, recombination of leucine-rich repeat modules is used to form variable lymphocyte receptors. Striking functional similarity of the cellular and humoral branches of these systems suggests similar driving forces underlying their development.