In zebrafish, the interaction of IL-4/13A with IL-4Rα on B cell surfaces promoted B cell proliferation and Ab production (68); in trout, treatment of head kidney cells with rIL-4/13 proteins downregulated IFN-γ (31), and upregulation of il4/13 paralogs was observed in the gills of Atlantic salmon following parasite infection (69). The gene discussed is IL4; the disease is parasitic infectious disease.