IL17A and bacterial infectious disease: Teleost IL-17A/F genes are mainly expressed in immune-related tissues (e.g. kidney, head kidney, and spleen) and mucosal tissues (e.g. gills, skin, and intestine) (23–28), and their gene expression can be modulated following bacterial infection and in vitro or in vivo stimulation of lipopolysaccharides (LPS), phytohaemagglutinin (PHA) and other immune stimuli (25–28).