A second class of immunoglobulin other than IgM present in the serum of a cartilaginous fish, the skate, Raja kenojei: isolation and characterization.

A second class of immunoglobulin, in addition to a pentameric immunoglobulin analogous to mammalian IgM, was found in the serum of a cartilaginous fish, the skate, Raja kenojei. The immunoglobulin was a non-covalently associated dimer with a sedimentation coefficient of 8.9S and a mol. wt of 320,000. Its heavy chain had a mol. wt of 45,000-50,000, smaller than that of the heavy chain (mol. wt 70,000) of the skate IgM, and had its own specific antigenic determinants. The immunoglobulin was synthesized in plasma cells different from those that produced the IgM. These results provide evidence that, contrary to the prevailing view, the divergence of immunoglobulin class occurred as early in evolution as the cartilaginous fish, a primitive vertebrate.