Effects of κ-carrageenan on the non-specific defense system of carp Cyprinus carpio

Carp which have received an intraperitoneal injection of kappa-carrageenan show a high survival rate after being challenged with pathogenic bacteria. In order to elucidate the mechanism underlying the immunoenhancement, we investigated its effects on the non-specific defense system of carp. kappa-Carrageenan had little influence on the activity of the alternative complement pathway or on the phagocytic and respiratory burst activities of head kidney phagocytes (HKP), yet it greatly elevated the migration of HKP to the peritoneal cavity. Since the migrated HKP (especially macrophages) showed a high phagocytic activity, it is highly probable that the early elimination of challenge bacteria by such mobilized and activated phagocytes was responsible for the high survival rate of the kappa-carrageenan-injected fish. It was also found that the phagocytic activity of the migrated head kidney macrophages was elevated not by resident peritoneal cells, but by leukocytes which had migrated to the peritoneal cavity. This indicates that the migrated leukocytes produced and secreted a factor(s) with macrophage activation potency.