Immunoglobulins(Igs),which bind antigens with high specificity,are important molecules in adaptive immune system of jawed vertebrates.This article summarizes all immunoglobulin isotypes reported in all vertebrate species,and focused on the current knowledge regarding isotypes and their loci in fish.Teleost fishes are found to express the isotypes of IgM,IgD,IgZ/IgT and a chimera IgMIgZ.In cartilaginous fish,three immunoglobulin isotypes,IgM,IgW and IgNAR have been identified,and IgW was reported also in lungfish.Furthermore,the genomic organization of IgH locus has been revealed in several species of teleost fish.The genome organization of IgH locus in channel catfish is a translocon arrangement,with three δ genes linked to a μ gene or pseudogene,and the loci in zebrafish,fugu and grass carp follow the pattern of genomic organization:Vn-Dn-Jn-Cζ-Dn-Jn-Cμ-Cδ.The organization of the IgH locus in rainbow trout is,in general,similar to that of zebrafish,fugu and grass carp,except that two VH genes are found downstream of the Cζ.The IgH loci organization in cartilaginous fishes are multiclusters,each consisting of a few gene segments(V_H-D-D-J_H-C_H)or(V_H-D-D-D-J_H-C_H).In different species of teleost fish,it is at least to some extent difficult or probably impossible to summarize a pattern in the expression of different Ig isotypes.Overall,these Ig isotypes are in general expressed in immune organs,such as thymus,head kidney and spleen.Despite the recent progress on the Ig genes and their loci in several species of teleost fish,many aspects such as the ontogeny of B lymphocytes,the pattern of antibody responses and the role of these different antibodies,and even the exact Ig gene composition and their loci may need to be further investigated either in different species or in greater detail.