Breaking the law: unconventional strategies for antibody diversification

Antibodies are essential components of adaptive immunity. A typical antibody repertoire comprises an enormous diversity of antigen-binding specificities, which are generated by the genetic processes of recombination and mutation. Accumulating evidence suggests that the immune system can exploit additional strategies to diversify the repertoire of antigen specificities. These unconventional mechanisms exclusively target the antigen-binding sites of immunoglobulins and include the insertion of large amino acid sequences, post-translational modifications, conformational heterogeneity and use of nonprotein cofactor molecules. Here, we describe the different unconventional routes for diversification of antibody specificities. Furthermore, we highlight how the immune system has a much greater level of adaptability and plasticity than previously anticipated, which goes far beyond that encoded in the genome or generated by the acquisition of somatic mutations. Diversification of the antibody repertoire is well known to be driven by genetic recombination and mutation. However, it is becoming apparent that other processes can also diversify antibody specificities. Here, Dimitrov and colleagues discuss these unconventional strategies for antibody diversification and consider why these extra strategies have evolved.