Effects of sodium alginate on the non-specific defence system of the common carp (Cyprinus carpioL.)

Carp which receive intraperitoneal injections of sodium alginate show a high survival rate after being challenged withEdwardsiella tarda. In order to elucidate the immunoenhancement by sodium alginate, its effects on the non-specific defence system of carp were investigated. Sodium alginate had little influence either on the activity of the alternative complement pathway or on the phagocytic and respiratory burst activities of head kidney phagocytes (HKP), yet it greatly enhanced the migration of HKP to the peritoneal cavity (the site of injection) and concurrently elevated their phagocytic activity. The number of phagocytes mobilised by sodium alginate was 2 to 50 times greater than that by the well-known peritoneal exudate cell-eliciting agents when injected at the same dose. Accordingly, it is highly probable that the early elimination of challenge bacteria by such mobilised and activated phagocytes was responsible for the high survival rate of the alginate-injected fish. In chemotaxis assays, it was revealed that sodium alginate stimulated some leucocyte subpopulation(s) within the peritoneal cavity to produce and/or secrete chemotactic factor(s), whilst concurrently enhancing the sensitivity of HKP to the factor(s).