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C-type lectins (CTLs) execute critical functions in multiple immune responses of crustaceans as a member of pattern recognition receptors (PRRs) family. In this study, a novel CTL was identified from the exoskeleton of the oriental river prawn Macrobrachium nipponense (Mn Lec3). The full-length cDNA of Mn Lec3 was 1150 bp with an open reading frame of 723 bp, encoding 240 amino acids. Mn Lec3 protein contained a signal peptide and one single carbohydrate-recognition domain (CRD). Mn Lec3 transcripts were widely distributed at the exoskeleton all over the body. Significant up-regulation of Mn Lec3 in exoskeleton after Aeromonas hydrophila challenged suggested the involvement of Mn Lec3 as well as the possible function of the exoskeleton in immune response. In vitro tests with recombinant Mn Lec3 protein (r Mn Lec3) manifested that it had polysaccharide binding activity, a wide spectrum of bacterial binding activity and agglutination activity only for tested Gram-negative bacteria (Escherichia coli , Vibrio anguillarum and A. hydrophila). Moreover, r Mn Lec3 significantly promoted phagocytic ability of hemocytes against A. hydrophila in vivo. What's more, Mn Lec3 interference remarkably impaired the survivability of the prawns when infected with A. hydrophila. Collectively, these results ascertained that Mn Lec3 derived from exoskeleton took an essential part in immune defense of the prawns against invading bacteria as a PRR. • A novel CTL (Mn Lec3) was identified from the exoskeleton of the oriental river prawn Macrobrachium nipponense. • Mn Lec3 plays a vital role in the immune response of the prawn. • 3.The exoskeleton might be more active in the immune response of the prawn by generating immunologic factors. [ABSTRACT FROM AUTHOR]