On Origin and Evolution of the Antibody Molecule

SIMPLE SUMMARY: Like many other molecules playing vital functions in animals, the antibody molecule possesses a complex structure with distinctive features. The structure of the basic unit, i.e., the immunoglobulin domain of very ancient origin is substantially simple. However, high complexity resides in the types and numbers of the domains composing the whole molecule. The emergence of the antibody molecule during evolution overturned the effectiveness of the organisms’ defense system. The particular organization of the coding genes, the mechanisms generating antibody diversity, and the plasticity of the overall protein structure, attest to an extraordinary successful evolutionary history. Here, we attempt to trace, across the evolutionary scale, the very early origins of the most significant features characterizing the structure of the antibody molecule and of the molecular mechanisms underlying its major role in recognizing an almost unlimited number of pathogens. ABSTRACT: The vertebrate immune system provides a powerful defense because of the ability to potentially recognize an unlimited number of pathogens. The antibody molecule, also termed immunoglobulin (Ig) is one of the major mediators of the immune response. It is built up from two types of Ig domains: the variable domain, which provides the capability to recognize and bind a potentially infinite range of foreign substances, and the constant domains, which exert the effector functions. In the last 20 years, advances in our understanding of the molecular mechanisms and structural features of antibody in mammals and in a variety of other organisms have uncovered the underlying principles and complexity of this fundamental molecule. One notable evolutionary topic is the origin and evolution of antibody. Many aspects have been clearly stated, but some others remain limited or obscure. By considering a wide range of prokaryotic and eukaryotic organisms through a literature survey about the topic, we have provided an integrated view of the emergence of antibodies in evolution and underlined the very ancient origins.