Phylogeny and Comparative Physiology of Mucosal Immunoglobulins

The evolution of mucosal immunoglobulins (Ig's) dates to the early origins of the adaptive immune system in jawed vertebrates. The concurrent emergence of genes encoding Ig heavy and light chains and the J chain in cartilaginous fish provided a mechanism for the polymerization of IgM, the primordial mucosal Ig. The emergence of IgT as a specialized mucosal Ig in bony fish was accompanied by the appearance of the polymeric Ig receptor, providing a mechanism for transport of polymeric Ig's into mucosal secretions. Divergence of the tetrapod lineage was accompanied by the emergence of IgX in amphibians, which evolved into IgA in birds and mammals (and possibly also reptiles). Although IgA is the predominant mucosal Ig in higher vertebrates, IgG, IgE, and even IgD contribute to the protection of mucosal surfaces. The evolution of mucosal Ig's, with their diverse structures and functions, has been a central theme in the development of mucosal homeostasis.