Evolution of immunoglobulins and their occurrence in vertebrates

The evolution of Ig occurred along the phylogenetic tree of the vertebrates (Kubo et al., 1973). All vertebrates seem to possess Ig (Nisonoff et al., 1975). A clear relation among the different classes of Ig in the diverse vertebrates is difficult to assess. Ig have not been shown conclusively in invertebrates (Marchalonis and Edelman, 1968). 

Classification is difficult since Ig of different classes may still have some sequence homology and, hence, cross-react, or have similar biological functions but different physicochemical properties. For example, IgY of birds (Leslie and Clem, 1972) which in biological respect resembles mammalian IgG, is in physicochemical respects quite different (e.g. contains an extra domain in the heavy chain). Tentative conclusions which may be drawn are (i) homologs of mammalian IgM seem to exist in all vertebrates (ii) IgA is found in mammals and birds, IgE in mammals and, perhaps, birds (iii) IgG is restricted to mammals only and (iv) vertebrates other than mammalia may produce other Ig such as IgY and IgN (Fig. 6.1). 

---