The cDNA sequence of the immunoglobulin light chain gene,AaIgL,in European eel (Anguilla anguilla) was cloned using reverse transcription PCR (RT-PCR).The full-length cDNA sequence of AaIgL is 1 016 bp,with a 714 bp open reading frame encoding 238 amino acids.The amplified DNA fragments were ligated into pET-his vector,transformed into E.coli BL21 and then induced by IPTG.The size of the expressed protein was 27 ku as estimated by SDS-PAGE electrophoresis and the recombinant proteins were strongly recognized by the serum anti-IgM of European eel detected by Western blotting,demonstrating that the fusion protein is expressed stably in E. coli BL21 with the form of inclusion bodies.Quantitative real-time RT-PCR analysis revealed a broad expression of AaIgL in a wide range of tissues,with the highest expression in spleen,followed by kidney and heart,and lowest in liver,muscles,gills,and intestine.After injection with goat IgG,AaIgL were obviously up-regulated in the spleen and the kidney,peaking at 7 d and 14 d,respectively.The peak expression level of AaIgL was much higher in the spleen and the expression recovered to normal level at 21 d in both spleen and kidney.Our results suggested that AaIgL might play an important role in fish’s immune defense system,and the spleen was the main organ for AaIgL expression after stimulation.