Systemic immune responses do not affect significant immune responses in the skin.

Fish skin plays an important role in defending against pathogens in water, primarily through the secretion of skin mucus containing various immune-related factors. Local immune responses in the skin activate systemic immune responses by inflammatory cytokines. However, it remains unclear whether immune responses in the skin occur after systemic immune responses caused by pathogen invasion into the fish body. This study aimed to clarify the relationship between systemic immune responses and skin responses after intraperitoneal injection of formalin-killed cells (FKC) of Vibrio anguillarum. Although systemic inflammatory responses were observed in the spleen after injection, expression changes in the skin did not show significant differences. In contrast, expression of hemoglobin subunit genes significantly increased in the skin after FKC injection, suggesting that erythrocytes infiltrate extravascularly.