Immunoglobulin Heavy Chain - in Fish

In Fish

Jawed fish appear to be the most primitive animals that are able to make antibodies like those described for mammals. However, fish do not have the same repertoire of antibodies that mammals possess. Three distinct Ig heavy chains have so far been identified in bony fish.

• The first identified was the μ (or mu) heavy chain that is present in all jawed fish and is the heavy chain for what is thought to be the primordial immunoglobulin. The resulting antibody, IgM, is secreted as a tetramer in teleost fish instead of the typical pentamer found in mammals and sharks.

• The heavy chain (δ) for IgD was identified initially from the channel catfish and Atlantic salmon and is now well documented for many teleost fish.

• The third teleost Ig heavy chain gene was identified very recently and does not resemble any of the heavy chains so far described for mammals. This heavy chain, identified in both rainbow trout (τ) and zebrafish (ζ), could potentially form a distinct antibody isotype (IgT or IgZ) that may precede IgM in evolutionary terms.

Similar to the situation observed for bony fish, three distinct Ig heavy chain isotypes have been identified in cartilaginous fish. With the exception of μ, these Ig heavy chain isotypes appear to be unique to cartilaginous fish. The resulting antibodies are designated IgW (also called IgX or IgNARC) and IgNAR ('immunoglobulin new antigen receptor'). The latter type is a heavy-chain antibody, an antibody lacking light chains, and can be used to produce single-domain antibodies, which are essentially the variable domain of an IgNAR.