Identification of a second immunoglobulin in the most primitive shark, the frill shark, Chlamydoselachus anguineus.

Three molecular forms of immunoglobulins: pentamer, dimer and monomer, were isolated from serum of the frill shark, Chlamydoselachus anguineus, the most primitive extant shark. A pentamer having an apparent mass of 900 kDa, consisting of 68 kDa heavy (H) chains and 22-24 kDa light (L) chains, was considered to be IgM because of its similarity to mammalian IgM in both molecular form and H chain molecular weight. The dimer and monomer with apparent masses of 300 kDa and 150 kDa, respectively, were composed of the same H chains of 45-50 kDa, a value smaller than that of the H chains from the pentamer, and light chains identical to those of the pentamer. The H chains of 68 kDa and 45-50 kDa represented individual epitopes and were synthesized by different plasma cells. We thus concluded that the frill shark has two distinct classes of immunoglobulins: one a pentameric IgM and the other a second class of immunoglobulin with dimeric and monomeric structure. The presence of the second Ig is of considerable interest with respect to antigen recognition and exclusion.