The class of immunoglobulin found in fish is
In immunology, antibody structure dictates that immunoglobulin fold with antiparallel beta-sandwich provides antigen binding. Therefore IgM is correct as it demonstrates immune recognition architecture, supported by mechanistic studies and conserved across related systems.
Ref: Janeway et al., Immunobiology, 9th Edition, Chapter 3, Antigen Recognition, discusses immunoglobulin fold, beta-sandwich of antiparallel beta sheets, antibody structure and humoral immunity explaining specificity.
