Phylogeny and comparative physiology of IgA

The mammalian mucosal immune system is the best characterized, and IgA is the major Ig of most human secretions. The cloning and sequencing of α-chain genes confirm the presence of IgA in several orders of eutherians and in marsupials, monotremes, and birds. The functional characteristics of secretory Igs, such as those defined by "second-order criteria," are found in IgM and other Ig isotypes. The α heavy chains of several primate species have been sequenced, revealing evolutionary developments. One of the most remarkable structural aspects of human IgA relates to the occurrence of two subclasses, IgA1 and IgA2, of which IgA1 has an elongated hinge region that is rich in proline residues. Among the anthropoid apes, the chimpanzee (. Pan troglodytes) and gorilla (. Gorilla gorilla) also have two IgA subclasses whose hinge regions are very similar to those of the respective human proteins. The hinge regions of gorilla and chimpanzee IgA1, like that of human IgA1, have the sequences that render them susceptible to cleavage by bacterial IgA1 proteases.